% \iffalse meta-comment
%
% Copyright (C) 1999 Frank Mittelbach, Chris Rowley, David Carlisle
% Copyright (C) 2004-2008 Frank Mittelbach, The LaTeX Project
% Copyright (C) 2009-2026
% The LaTeX Project and any individual authors listed elsewhere
% in this file.
%
% This file is part of the LaTeX base system.
% -------------------------------------------
%
% It may be distributed and/or modified under the
% conditions of the LaTeX Project Public License, either version 1.3c
% of this license or (at your option) any later version.
% The latest version of this license is in
%    https://www.latex-project.org/lppl.txt
% and version 1.3c or later is part of all distributions of LaTeX
% version 2008 or later.
%
% This file has the LPPL maintenance status "maintained".
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% The list of all files belonging to the LaTeX base distribution is
% given in the file `manifest.txt'. See also `legal.txt' for additional
% information.
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% The list of derived (unpacked) files belonging to the distribution
% and covered by LPPL is defined by the unpacking scripts (with
% extension .ins) which are part of the distribution.
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% \fi
%
% \iffalse
%
%%% From File: ltcmd.dtx
%
%    \begin{macrocode}
\def\ltcmdversion{v1.3n}
\def\ltcmddate{2026-07-28}
%    \end{macrocode}
%
%<*driver>
% \fi
\ProvidesFile{ltcmd.dtx}
  [\ltcmddate\space \ltcmdversion\space
   LaTeX Kernel (Document commands)]
% \iffalse
\documentclass{l3in2edoc}
\GetFileInfo{ltcmd.dtx}
\title{\filename}
\date{\filedate}
\author{Frank Mittelbach, Chris Rowley, David Carlisle, \LaTeX{} Project Team}
\begin{document}
  \maketitle
  \DocInput{ltcmd.dtx}
\end{document}
%</driver>
% \fi
%
% \section{Creating document commands}
%
% \changes{v1.0a}{2020/11/20}{Initial version derived from \texttt{xparse.dtx}}
% \changes{v1.2a}{2023/08/19}{Removed commands that should have remained
%   only in \texttt{xparse.dtx}}
% \changes{v1.3j}{2026-02-25}{Add documentation as part of \texttt{cmdguide}
%   work}
% \changes{v1.3m}{2026-06-01}{Switch to US spelling for macro names}
%
% \subsection{Overview}
%
% Creating document commands and environments using the \LaTeX3 toolset is based
% around the idea that a common set of descriptions can be used to cover almost
% all argument types used in real documents. Thus parsing is reduced to a simple
% description of which arguments a command takes: this description provides the
% \enquote{glue} between the document syntax and the implementation of the
% command.
%
% First, we will describe the argument types, then move on to explain how these
% can be used to create both document commands and environments. Various more
% specialized features are then described, which allow an even richer application
% of a simple interface set up.
%
% The details here are intended to help users create document commands in
% general. More technical detail, suitable for \TeX{} programmers, is included
% in \texttt{interface3}.
%
% \subsection{Describing argument types}
%
% In order to allow each argument to be defined independently, the parser does
% not simply need to know the number of arguments for a function, but also the
% nature of each one. This is done by constructing an \emph{argument
% specification}, which defines the number of arguments, the type of each
% argument and any additional information needed for the parser to read the user
% input and properly pass it through to internal functions.
%
% The basic form of the argument specifier is a list of letters, where each
% letter defines a type of argument. As will be described below, some of the
% types need additional information, such as default values. The argument types
% can be divided into two, those which define arguments that are mandatory
% (potentially raising an error if not found) and those which define optional
% arguments. The mandatory types
% \begin{itemize}
%   \item[\texttt{m}] A standard mandatory argument, which can either be a single token
%     alone or multiple tokens surrounded by curly braces |{}|. Regardless of the
%     input, the argument will be passed to the internal code without the outer
%     braces. This is the type specifier for a normal \TeX{} argument.
%   \item[\texttt{r}] Given as \texttt{r}\meta{token1}\meta{token2}, this denotes a
%     \enquote{required} delimited argument, where the delimiters are
%     \meta{token1} and \meta{token2}. If the opening delimiter \meta{token1} is
%     missing, the default marker \cs{NoValue} will be inserted after a suitable
%     error.
%   \item[\texttt{R}] Given as \texttt{R}\meta{token1}\meta{token2}\marg{default},
%     this is a \enquote{required} delimited argument as for~\texttt{r},
%     but it has a user-definable recovery \meta{default} instead of
%     \cs{NoValue}.
%   \item[\texttt{v}] Reads an argument \enquote{verbatim}, between the following
%     character and its next occurrence, in a way similar to the argument
%     of the \LaTeXe{} command \cs{verb}. Thus a \texttt{v}-type argument
%     is read between two identical characters, which cannot be any of |%|, |\|,
%     |#|, |{|, |}| or \verb*| |. The verbatim argument can also be enclosed
%     between braces, |{| and |}|. A command with a verbatim argument will
%     produce an error when it appears within an argument of another command.
%   \item[\texttt{b}] Only suitable in the argument specification of an
%     environment, it denotes the body of the environment, between
%     |\begin|\marg{environment} and |\end|\marg{environment}.  See
%     Section~\ref{sec:cmd:body} for details.
%   \item[\texttt{c}] Only suitable in the argument specification of an
%     environment, it denotes collection of the environment verbatim, between
%     |\begin|\marg{environment} and |\end|\marg{environment}.  See
%     Section~\ref{sec:cmd:verbenv} for details.
% \end{itemize}
% The types which define optional arguments are:
% \begin{itemize}
%   \item[\texttt{o}] A standard \LaTeX{} optional argument, surrounded with square
%     brackets, which will supply the special \cs{NoValue} marker if not given
%     (as described later).
%   \item[\texttt{d}] Given as \texttt{d}\meta{token1}\meta{token2}, an optional
%     argument which is delimited by \meta{token1} and \meta{token2}. As with
%     \texttt{o}, if no value is given the special marker \cs{NoValue} is
%     returned.
%   \item[\texttt{O}] Given as \texttt{O}\marg{default}, is like \texttt{o}, but returns
%     \meta{default} if no value is given.
%   \item[\texttt{D}] Given as \texttt{D}\meta{token1}\meta{token2}\marg{default}, it is
%     as for \texttt{d}, but returns \meta{default} if no value is given.
%     Internally, the \texttt{o}, \texttt{d} and \texttt{O} types are
%     short-cuts to an appropriated-constructed \texttt{D} type argument.
%   \item[\texttt{s}] An optional star, which will result in a value
%     \cs{BooleanTrue} if a star is present and \cs{BooleanFalse}
%     otherwise (as described later).
%   \item[\texttt{t}] An optional \meta{token}, which will result in a value
%     \cs{BooleanTrue} if \meta{token} is present and \cs{BooleanFalse}
%     otherwise. Given as \texttt{t}\meta{token}.
%   \item[\texttt{e}] Given as \texttt{e}\marg{tokens}, a set of optional
%     \emph{embellishments}, each of which requires a \emph{value}.
%     If an embellishment is not present, \cs{NoValue} is returned.  Each
%     embellishment gives one argument, ordered as for the list of
%     \meta{tokens} in the argument specification.  All \meta{tokens}
%     must be distinct.
%   \item[\texttt{E}] As for \texttt{e} but returns one or more \meta{defaults}
%     if values are not given: \texttt{E}\marg{tokens}\marg{defaults}. See
%     Section~\ref{sec:cmd:embellishment} for more details.
% \end{itemize}
%
% \subsection{Modifying argument descriptions}
%
% In addition to the argument \emph{types} discussed above, the argument
% description also gives special meaning to three other characters.
%
% First, \texttt{+} is used to make an argument long (to accept paragraph
% tokens). In contrast to \cs{newcommand}, this applies on an
% argument-by-argument basis. So modifying the example to
% \enquote{\texttt{s o o +m O{default}}}
% means that the mandatory argument is now \cs{long}, whereas the optional
% arguments are not.
%
% Secondly, \texttt{!} is used to control whether spaces are allowed before
% optional arguments. There are some subtleties to this, as \TeX{} itself
% has some restrictions on where spaces can be \enquote{detected}: more detail
% is given in Section~\ref{sec:cmd:opt-space}.
%
% Thirdly, \texttt{=} is used to declare that the following argument should
% be interpreted as a series of keyvals. See Section~\ref{sec:cmd:keyval}
% for more details.
%
% Finally, the character \texttt{>} is used to declare so-called
% `argument processors', which can be used to modify the contents of an
% argument before it is passed to the macro definition. The use of argument
% processors is a somewhat advanced topic, (or at least a less commonly used
% feature) and is covered in Section~\ref{sec:cmd:processors}.
%
% \subsection{Creating document commands and environments}
%
% \begin{function}
%   {
%     \NewDocumentCommand,
%     \RenewDocumentCommand,
%     \ProvideDocumentCommand,
%     \DeclareDocumentCommand
%   }
%   \begin{syntax}
%     \cs{NewDocumentCommand} \Arg{cmd} \Arg{arg spec} \Arg{code}
%   \end{syntax}
% \end{function}
% This family of commands are used to create a \meta{cmd}. The argument
% specification for the function is given by \meta{arg spec}, and the command
% uses the \meta{code} with |#1|, |#2|, etc.\ replaced by the arguments found
% by the parser.
%
% An example:
% \begin{Verbatim}[gobble = 3]
%    \NewDocumentCommand\chapter{s o m}
%      {%
%        \IfBooleanTF{#1}%
%          {\typesetstarchapter{#3}}%
%          {\typesetnormalchapter{#2}{#3}}%
%      }
% \end{Verbatim}
% would be a way to define a \cs{chapter} command which would essentially behave
% like the current \LaTeXe{} command (except that it would accept an optional
% argument even when a \texttt{*} was parsed). The \cs{typesetnormalchapter}
% could test its first argument for being \cs{NoValue} to see if an optional
% argument was present. (See Section~\ref{sec:cmd:special} for details of
% \cs{IfBooleanTF} and testing for \cs{NoValue}.)
%
% The difference between the \cs{New...} \cs{Renew...}, \cs{Provide...}
% and \cs{Declare...} versions is the behavior if \meta{cmd} is already
% defined.
% \begin{itemize}
%  \item \cs{NewDocumentCommand} will issue an error if \meta{cmd}
%    has already been defined.
%  \item \cs{RenewDocumentCommand} will issue an error if \meta{cmd}
%    has not previously been defined.
%  \item \cs{ProvideDocumentCommand} creates a new definition for
%    \meta{cmd} only if one has not already been given.
%  \item \cs{DeclareDocumentCommand} will always create the new
%    definition, irrespective of any existing \meta{cmd} with the
%    same name.  This should be used sparingly.
% \end{itemize}
%
% If the \meta{cmd} can't be provided as a single token but needs
% \enquote{constructing}, you can use \cs{ExpandArgs} as explained in
% Section~\ref{sec:preconstructing-csnames} which also gives an example
% in which this is needed.
%
% \begin{function}
%   {
%     \NewDocumentEnvironment,
%     \RenewDocumentEnvironment,
%     \ProvideDocumentEnvironment,
%     \DeclareDocumentEnvironment
%   }
%   \begin{syntax}
%     \cs{NewDocumentEnvironment} \Arg{env} \Arg{arg spec} \Arg{beg-code} \Arg{end-code}
%   \end{syntax}
% \end{function}
% These commands work in the same way as \cs{NewDocumentCommand}, etc.\@, but
% create environments (\cs{begin}\Arg{env} \ldots{}
% \cs{end}\Arg{env}). Both the \meta{beg-code} and \meta{end-code}
% may access the arguments as defined by \meta{arg spec}. The arguments will be
% given following \cs{begin}\Arg{env}. Any spaces at the start and end of the
% \Arg{env} are removed before the definition takes place, thus
% \begin{Verbatim}[gobble = 3]
%    \NewDocumentEnvironment{foo}
% \end{Verbatim}
% and
% \begin{Verbatim}[gobble = 3]
%    \NewDocumentEnvironment{ foo }
% \end{Verbatim}
% both create the same \enquote{\texttt{foo}} environment.
%
% \subsection{Optional arguments}
% \label{sec:cmd:opt}
%
% In contrast to commands created using \LaTeXe{}'s \cs{newcommand}, optional
% arguments created using \cs{NewDocumentCommand} may safely be nested. Thus for
% example, following
% \begin{Verbatim}[gobble = 3]
%    \NewDocumentCommand\foo{om}{I grabbed `#1' and `#2'}
%    \NewDocumentCommand\baz{o}{#1-#1}
% \end{Verbatim}
% using the command as
% \begin{Verbatim}[gobble = 3]
%    \foo[\baz[stuff]]{more stuff}
% \end{Verbatim}
% will print
% \begin{quote}
% I grabbed `stuff-stuff' and `more stuff'
% \end{quote}
% This is particularly useful when placing a command with an optional argument
% \emph{inside} the optional argument of a second command.
%
% When an optional argument is followed by a mandatory argument with the same
% delimiter, the parser issues a warning because the optional argument could not
% be omitted by the user, thus becoming in effect mandatory. This can apply to
% \texttt{o}, \texttt{d}, \texttt{O}, \texttt{D}, \texttt{s}, \texttt{t},
% \texttt{e}, and \texttt{E} type arguments followed by \texttt{r} or
% \texttt{R}-type required arguments.
%
% The default for \texttt{O}, \texttt{D} and \texttt{E} arguments can be
% the result of grabbing another argument. Thus for example
% \begin{Verbatim}[gobble = 3]
%    \NewDocumentCommand\foo{O{#2} m}
% \end{Verbatim}
% would use the mandatory argument as the default for the leading optional
% one.
%
% \subsection{Spacing and optional arguments}
% \label{sec:cmd:opt-space}
%
% \TeX{} will find the first argument after a function name irrespective of any
% intervening spaces. This is true for both mandatory and optional arguments. So
% |\foo[arg]| and \verb*|\foo [arg]| are equivalent. Spaces are also ignored when
% collecting arguments up to the last mandatory argument to be collected (as it
% must exist). So after
% \begin{Verbatim}[gobble = 3]
%    \NewDocumentCommand\foo{m o m}{ ... }
% \end{Verbatim}
% the user input |\foo{arg1}[arg2]{arg3}| and \verb*|\foo{arg1} [arg2] {arg3}|
% will both be parsed in the same way.
%
% The behavior of optional arguments \emph{after} any mandatory arguments is
% selectable. The standard settings will allow spaces here, and thus with
% \begin{Verbatim}[gobble = 3]
%    \NewDocumentCommand\foobar{m o}{ ... }
% \end{Verbatim}
% both |\foobar{arg1}[arg2]| and \verb*|\foobar{arg1} [arg2]| will find an
% optional argument. This can be changed by giving the modified |!| in the
% argument specification:
% \begin{Verbatim}[gobble = 3]
%    \NewDocumentCommand\foobar{m !o}{ ... }
% \end{Verbatim}
% where \verb*|\foobar{arg1} [arg2]| will not find an optional argument.
%
% There is one subtlety here due to the difference in handling by \TeX{} of
% `control symbols', where the command name is made up of a single
% character, such as `\texttt{\textbackslash\textbackslash}'.
% Spaces are not ignored by \TeX{} here,
% and thus it is possible to require an optional argument directly follow such a
% command. The most common example is the use of \texttt{\textbackslash\textbackslash}
% in \pkg{amsmath} environments, which in the terms here would be defined as
% \begin{Verbatim}[gobble = 3]
%    \NewDocumentCommand\\{!s !o}{ ... }
% \end{Verbatim}
%
% Also notable when using optional arguments in the last position is that \TeX{}
% will necessarily look ahead for the argument opening token. This means that
% the value of |\inputlineno| will be `out by one' if such a trailing optional
% argument is \emph{not} present and the command ends a line; it will be one
% greater than the line number containing the last mandatory argument.
%
% \subsection{`Embellishments'}
% \label{sec:cmd:embellishment}
%
% The \texttt{E}-type argument allows one default value per test token. This is
% achieved by giving a list of defaults for each entry in the list, for example:
% \begin{Verbatim}[gobble = 3]
%    E{^_}{{UP}{DOWN}}
% \end{Verbatim}
% If the list of default values is \emph{shorter} than the list of test tokens,
% the special \cs{NoValue} marker will be returned (as for the \texttt{e}-type
% argument). Thus for example
% \begin{Verbatim}[gobble = 3]
%    E{^_}{{UP}}
% \end{Verbatim}
% has default \texttt{UP} for the |^| test character, but will return the
% \cs{NoValue} marker as a default for |_|. This allows mixing of explicit
% defaults with testing for missing values.
%
% \subsection{Testing special values}
% \label{sec:cmd:special}
%
% Optional arguments make use of dedicated variables to return information about
% the nature of the argument received.
%
% \begin{function}{\IfNoValueTF, \IfNoValueT, \IfNoValueF}
%   \begin{syntax}
%     \cs{IfNoValueTF} \Arg{arg} \Arg{true code} \Arg{false code}
%   \end{syntax}
% \end{function}
% The \cs{IfNoValue(TF)} tests are used to check if \meta{argument} (|#1|,
% |#2|, \emph{etc.}) is the special \cs{NoValue} marker. For example
% \begin{Verbatim}[gobble = 3]
%    \NewDocumentCommand\foo{o m}
%      {%
%        \IfNoValueTF {#1}%
%          {\DoSomethingJustWithMandatoryArgument{#2}}%
%          {\DoSomethingWithBothArguments{#1}{#2}}%
%      }
% \end{Verbatim}
% will use a different internal function if the optional argument
% is given than if it is not present.
%
% Note that three tests are available, depending on which outcome
% branches are required: \cs{IfNoValueTF}, \cs{IfNoValueT} and
% \cs{IfNoValueF}.
%
% As the \cs{IfNoValue(TF)} tests are expandable, it is possible to
% test these values later, for example at the point of typesetting or
% in an expansion context.
%
% When two optional arguments follow each other (a syntax we typically
% discourage), it can make sense to allow users of the command to
% specify only the second argument by providing an empty first
% argument.
% If you wish to test if an argument is blank or not, but are not
% concerned with distinguishing an entirely absent argument from
% an empty one, use the |O|~type specifier with the
% conditional \cs{IfBlankTF} (described below).
%
% \begin{function}{\IfValueTF, \IfValueT, \IfValueF}
%   \begin{syntax}
%     \cs{IfValueTF} \Arg{arg} \Arg{true code} \Arg{false code}
%   \end{syntax}
% \end{function}
% The reverse form of the \cs{IfNoValue(TF)} tests are also available
% as \cs{IfValue(TF)}. The context will determine which logical
% form makes the most sense for a given code scenario.
%
% \begin{function}{\IfBlankTF, \IfBlankT, \IfBlankF}
%   \begin{syntax}
%     \cs{IfBlankTF} \Arg{arg} \Arg{true code} \Arg{false code}
%   \end{syntax}
% \end{function}
%
% The \cs{IfNoValueTF} command chooses the \meta{true code} if the optional argument has not
% been used at all (and it returns the special \cs{NoValue}
% marker), but not if it has been given an empty value. In contrast
% \cs{IfBlankTF} returns true if its argument is either truly empty or
% only contains one or more normal blanks.
% For example
% \begin{Verbatim}[gobble = 3]
%    \NewDocumentCommand\foo{m!o}{\par #1:
%      \IfNoValueTF{#2}
%        {No optional}%
%        {%
%          \IfBlankTF{#2}
%            {Blanks in or empty}%
%            {Real content in}%
%        }%
%      \space argument!}
%    \foo{1}[bar] \foo{2}[  ] \foo{3}[] \foo{4}[\space] \foo{5} [x]
% \end{Verbatim}
% results
% in the following output:
% \begin{quote}
%   \NewDocumentCommand\foo{m!o}{\par #1:
%     \IfNoValueTF{#2}{No optional}%
%                 {\IfBlankTF{#2}{Blanks in or empty}%
%                   {Real content in}}%
%                 \space argument!}
%   \foo{1}[bar] \foo{2}[  ] \foo{3}[] \foo{4}[\space] \foo{5} [x]
% \end{quote}
%
% Note that the \cs{space} in (4) is considered real content---because it is
% a command and not a \enquote{space} character---even though it results
% in producing a space. You can also observe in (5) the effect of the
% \texttt{!} specifier, preventing the last \cs{foo}
% from interpreting \texttt{[x]} as its optional argument.
%
% \begin{variable}{\BooleanTrue, \BooleanFalse}
% \end{variable}
% The \texttt{true} and \texttt{false} flags set when searching for
% an optional character (using \texttt{s} or \texttt{t\meta{char}}) have
% names which are accessible outside of code blocks.
%
% \begin{function}{\IfBooleanTF, \IfBooleanT, \IfBooleanF}
%   \begin{syntax}
%     \cs{IfBooleanTF} \Arg{arg} \Arg{true code} \Arg{false code}
%   \end{syntax}
% \end{function}
% Used to test if \meta{argument} (|#1|, |#2|, \emph{etc.}) is
% \cs{BooleanTrue} or \cs{BooleanFalse}. For example
% \begin{Verbatim}[gobble = 3]
%    \NewDocumentCommand\foo{sm}
%      {%
%        \IfBooleanTF {#1}%
%          {\DoSomethingWithStar{#2}}%
%          {\DoSomethingWithoutStar{#2}}%
%      }
% \end{Verbatim}
% checks for a star as the first argument, then chooses the action to
% take based on this information.
%
% \subsection{Auto-converting to key--value format}
% \label{sec:cmd:keyval}
%
% Some document commands have a long history of accepting a `free text' optional
% argument, for example \cs{caption} and the sectioning commands \cs{section},
% etc. Introducing more sophisticated (keyval) options to these commands
% therefore needs a method to interpret the optional argument \emph{either} as
% free text \emph{or} as a series of keyvals. This needs to take place
% during argument grabbing as there is a need for careful treatment of
% braces to obtain the correct result.
%
% The \texttt{=} modifier is available to allow \pkg{ltcmd} to correctly
% implement this process. The modifier guarantees that the argument will be
% passed to further code as a series of keyvals. To do that, the \texttt{=}
% should be followed by an argument containing the default key name. This is used
% as the key in a key--value pair \emph{if} the \enquote{raw} argument does \emph{not}
% have the correct form to be interpreted as a set of keyvals.
%
% Taking \cs{caption} as an example, with the demonstration implementation
% \begin{Verbatim}[gobble = 3]
%    \DeclareDocumentCommand\caption{s ={short-text} +O{#3} +m}
%      {%
%        \showtokens{Grabbed arguments:^^J(#2)^^Jand^^J(#3)}%
%      }
% \end{Verbatim}
% the default key name is \texttt{short-text}. When the command \cs{caption} is
% then used, if the optional argument is free text such as
% \begin{Verbatim}[gobble = 3]
%    \caption[Some short text]{A much longer and more detailed text for
%      demonstration purposes}
% \end{Verbatim}
% then the output will be
% \begin{Verbatim}[gobble = 3]
%    Grabbed arguments:
%    (short-text={Some short text})
%    and
%    (A much longer and more detailed text for demonstration purposes)
% \end{Verbatim}
% On the other hand, if the caption is given with a keyval-form argument
% \begin{Verbatim}[gobble = 3]
%    \caption[label = cap:demo]%
%      {A much longer and more detailed text for demonstration purposes}
% \end{Verbatim}
% then this will be respected
% \begin{Verbatim}[gobble = 3]
%    Grabbed arguments:
%    (label = cap:demo)
%    and
%    (A much longer and more detailed text for demonstration purposes)
% \end{Verbatim}
%
% Interpretation as keyval form is determined by the presence of \texttt{=}
% characters within the argument. Those in inline math mode (enclosed within
% \verb|$...$| or \verb|\(...\)|) are ignored. An argument can be forced to be
% read as keyvals by including an empty entry at the start
% \begin{Verbatim}[gobble = 3]
%    \caption[=,This is now a keyval]%
%    % ...
%    \caption[This is not $=$ keyval]%
% \end{Verbatim}
%
% This empty entry is \emph{not} passed to the underlying code, so will not lead
% to issues with keyval parsers that do not allow an empty key name. Any text-mode
% \texttt{=} signs will need to be braced to avoid being misinterpreted: this
% is likely most conveniently handled by bracing the entire argument
% \begin{Verbatim}[gobble = 3]
%    \caption[{Not = to a keyval!}]%
% \end{Verbatim}
% which will be passed correctly as
% \begin{Verbatim}[gobble = 3]
%    Grabbed arguments:
%    (short-text = {Not = to a keyval!})
% \end{Verbatim}
%
% If the argument is completely blank, the conversion results in an empty keyval
% list, not \verb*|short-text = |. This reflects the fact that with a move toward
% keyval processing, an empty argument is best modelled as an empty keyval list.
% If the user does want an empty classical argument, using |[{}]| will work
% with both the new processor code and older formats.
%
% \subsection{Argument processors}
% \label{sec:cmd:processors}
%
% Argument processor are applied to an argument \emph{after} it has been grabbed
% by the underlying system but before it is passed to \meta{code}. An argument
% processor can therefore be used to regularize input at an early stage, allowing
% the internal functions to be completely independent of input form. Processors
% are applied to user input and to default values for optional arguments, but
% \emph{not} to the special \cs{NoValue} marker.
%
% Each argument processor is specified by the syntax \texttt{>}\marg{processor}
% in the argument specification. Processors are applied from right to left, so
% that
% \begin{Verbatim}[gobble = 3]
%    >{\ProcessorB} >{\ProcessorA} m
% \end{Verbatim}
% would apply \cs{ProcessorA} followed by \cs{ProcessorB} to the tokens grabbed
% by the \texttt{m} argument.
%
% \begin{function}{\SplitArgument}
%   \begin{syntax}
%     \cs{SplitArgument} \Arg{number} \Arg{token(s)}
%   \end{syntax}
% \end{function}
% This processor splits the argument given at each occurrence of the
% \meta{tokens} up to a maximum of \meta{number} tokens (thus
% dividing the input into $\text{\meta{number}} + 1$ parts).
% An error is given if too many \meta{tokens} are present in the
% input. The processed input is placed inside
% $\text{\meta{number}} + 1$ sets of braces for further use.
% If there are fewer than \Arg{number} of \Arg{tokens} in the argument
% then \cs{NoValue} markers are added at the end of the processed
% argument.
% \begin{Verbatim}[gobble = 3]
%    \NewDocumentCommand\foo{>{\SplitArgument{2}{;}} m}
%      {\InternalFunctionOfThreeArguments#1}
% \end{Verbatim}
% If only a single character \meta{token} is used for the split, any
% category code $13$ (active) character matching the \meta{token} will
% be replaced before the split takes place.
% Spaces are trimmed at each end of each item parsed.
%
% The \texttt{E} argument type is somewhat special, because with a single
% \texttt{E} in the command declaration you may end up with several
% arguments in a command (one formal argument per embellishment token).
% Therefore, when an argument processor is applied to an
% \texttt{e}/\texttt{E}-type
% argument, all the arguments pass through that processor before being fed
% to the \meta{code}.  For example, this command
% \begin{Verbatim}[gobble = 3]
%    \NewDocumentCommand\foo{ >{\TrimSpaces} e{_^} }
%      { [#1](#2) }
% \end{Verbatim}
% applies \cs{TrimSpaces} to both arguments.
%
% \begin{function}{\SplitList}
%   \begin{syntax}
%     \cs{SplitList} \Arg{token(s)}
%   \end{syntax}
% \end{function}
% This processor splits the argument given at each occurrence of the
% \meta{token(s)} where the number of items is not fixed. Each item is
% then wrapped in braces within |#1|. The result is that the
% processed argument can be further processed using a mapping function
% (see below).
% \begin{Verbatim}[gobble = 3]
%    \NewDocumentCommand\foo{>{\SplitList{;}} m}
%      {\MappingFunction#1}
% \end{Verbatim}
% If only a single character \meta{token} is used for the split, it will
% take account of the possibility that the \meta{token} has been made active
% (category code~$13$) and will split at such tokens. 
% Spaces are trimmed at each end of each item parsed. Exactly one set
% of braces will be stripped if an entire item is surrounded by them,
% i.e.~the following inputs and outputs result (each separate item as
% a brace group).
% \begin{Verbatim}[gobble = 3]
%    a      ==> {a}
%    {a}    ==> {a}
%    {a}b   ==> {{a}b}
%    a,b    ==> {a}{b}
%    {a},b  ==> {a}{b}
%    a,{b}  ==> {a}{b}
%    a,{b}c ==> {a}{{b}c}
% \end{Verbatim}
%
% \begin{function}{\ProcessList}
%   \begin{syntax}
%     \cs{ProcessList} \Arg{list} \Arg{token(s)}
%   \end{syntax}
% \end{function}
% To support \cs{SplitList}, the function \cs{ProcessList} is available
% to apply \meta{tokens} to every entry in a \meta{list}. The
% \meta{tokens} can be arbitrary contents that should expect one argument
% after it: the list entry. For example
% \begin{Verbatim}[gobble = 3]
%    \NewDocumentCommand\foo{>{\SplitList{;}} m}
%      {\ProcessList{#1}{\SomeDocumentCommand}}
% \end{Verbatim}
% or
% \begin{Verbatim}[gobble = 3]
%    \NewDocumentCommand\foo{>{\SplitList{;}} m}
%      {\ProcessList{#1}{Abc \SomeDocumentCommand}}
% \end{Verbatim}
%
% \begin{function}{\ReverseBoolean}
%   \begin{syntax}
%     \cs{ReverseBoolean}
%   \end{syntax}
% \end{function}
% This processor reverses the logic of \cs{BooleanTrue} and
% \cs{BooleanFalse}, so that the example from earlier would become
% \begin{Verbatim}[gobble = 3]
%    \NewDocumentCommand\foo{>{\ReverseBoolean} s m}
%      {%
%        \IfBooleanTF#1%
%          {\DoSomethingWithoutStar{#2}}%
%          {\DoSomethingWithStar{#2}}%
%      }
% \end{Verbatim}
%
% \begin{function}{\TrimSpaces}
%   \begin{syntax}
%     \cs{TrimSpaces}
%   \end{syntax}
% \end{function}
% Removes any leading and trailing spaces (tokens with character code~$32$
% and category code~$10$) for the ends of the argument. Thus for example
% declaring a function
% \begin{Verbatim}[gobble = 3]
%    \NewDocumentCommand\foo{>{\TrimSpaces} m}
%      {\showtokens{#1}}
% \end{Verbatim}
% and using it in a document as
% \begin{flushleft}
%   \verb=  =\verb*=\foo{ hello world }=
% \end{flushleft}
% will show `\verb*=hello world=' at the terminal, with the space at each
% end removed. \cs{TrimSpaces} will remove multiple spaces from the ends of
% the input in cases where these have been included such that the standard
% \TeX{} conversion of multiple spaces to a single space does not apply.
%
% \subsection{Body of an environment}
% \label{sec:cmd:body}
%
% While environments |\begin|\marg{environment}\ \dots{}\,|\end|\marg{environment}
% are typically used in cases where the code implementing the \meta{environment}
% does not need to access the contents of the environment (its \enquote{body}),
% it is sometimes useful to have the body as a standard argument.
%
% This is achieved by ending the argument specification with~\texttt{b}, which is
% a dedicated argument type for this situation. For instance
% \begin{Verbatim}[gobble = 3]
%    \NewDocumentEnvironment{twice}{O{\ttfamily} +b}
%      {#2#1#2} {}
%    \begin{twice}[\itshape]
%      Hello world!
%    \end{twice}
% \end{Verbatim}
% typesets `Hello world!{\itshape Hello world!}'.
%
% The prefix |+| is used to allow multiple paragraphs in the environment's body.
% Argument processors can also be applied to \texttt{b}~arguments. By default,
% spaces are trimmed at both ends of the body: in the example there would
% otherwise be spaces coming from the ends the lines after |[\itshape]| and
% |world!|. Putting the prefix |!| before \texttt{b} suppresses space-trimming.
%
% When \texttt{b} is used in the argument specification, the last
% argument of the environment declaration (e.g.,
% \cs{NewDocumentEnvironment}), which consists of an \meta{end code} to
% insert at |\end|\marg{environment}, is redundant since one can simply
% put that code at the end of the \meta{start code}. Nevertheless this
% (empty) \meta{end code} must be provided.
%
% Environments that use this feature can be nested.
%
% \subsection{Fully-expandable document commands\label{sec:ltcmd:expandable}}
%
% Document commands created using \cs{NewDocumentCommand}, etc.\@, are normally
% created so that they do not expand unexpectedly. This is done using engine
% features, so is more powerful than \LaTeXe{}'s \cs{protect} mechanism. There
% are \emph{very rare} occasion when it may be useful to create functions using a
% expansion-only grabber. This imposes a number of restrictions on the
% nature of the arguments accepted by a function, and the code it implements.
% This facility should only be used when \emph{necessary}.
%
% \begin{function}
%   {
%     \NewExpandableDocumentCommand,
%     \RenewExpandableDocumentCommand,
%     \ProvideExpandableDocumentCommand,
%     \DeclareExpandableDocumentCommand
%   }
%   \begin{syntax}
%     \cs{NewExpandableDocumentCommand} \Arg{cmd} \Arg{arg spec} \Arg{code}
%   \end{syntax}
% \end{function}
% This family of commands is used to create a document-level \meta{cmd},
% which will grab its arguments in a fully-expandable manner. The
% argument specification for the function is given by \meta{arg spec},
% and the \meta{cmd} will execute \meta{code}. In  general, \meta{code} will
% also be fully expandable, although it is possible that this will
% not be the case (for example, a function for use in a table might
% expand so that \cs{omit} is the first non-expandable non-space token).
%
% Parsing arguments by pure expansion imposes a number of restrictions on
% both the type of arguments that can be read and the error checking
% available:
% \begin{itemize}
%   \item The last argument (if any are present) must be one of the
%     mandatory types \texttt{m}, \texttt{r} or \texttt{R}.
%   \item The \enquote{verbatim} argument type \texttt{v} is not available.
%   \item Argument processors (using \texttt{>}) are not available.
%   \item It is not possible to differentiate between, for example
%     |\foo[| and |\foo{[}|: in both cases the \texttt{[} will be
%     interpreted as the start of an optional argument. As a
%     result, checking for optional arguments is less robust than
%     in the standard version.
% \end{itemize}
%
% \subsection{Commands at the start of tabular cells}
%
% Creating commands that are used at the start of tabular cells imposes
% some restrictions on the underlying implementation. The standard \LaTeX{}
% tabular environments (\texttt{tabular}, etc.)  use a mechanism which requires
% that any command wrapping \cs{multicolumn} or similar must be
% \enquote{expandable}. This is \emph{not} the case for commands created using
% \cs{NewDocumentCommand}, etc., which as detailed in
% Section~\ref{sec:ltcmd:expandable} use an engine feature which prevents
% such \enquote{expansion}. Therefore, to create such wrappers for use at the start
% of tabular cells, you must use \cs{NewExpandableDocumentCommand}, for example
% \begin{Verbatim}[gobble = 3]
%    \NewExpandableDocumentCommand\MyMultiCol{m}{\multicolumn{3}{c}{#1}}
%    \begin{tabular}{lcr}
%    a & b & c \\
%    \MyMultiCol{stuff} \\
%    \end{tabular}
% \end{Verbatim}
%
% \subsection{Using the verbatim argument types}
%
% As described above, the \texttt{v}-type argument may be viewed as similar to
% \cs{verb}. Before looking at exactly what that means, it is important to
% highlight some key differences. Most notably, \emph{grabbing} a verbatim-like
% argument is separate from \emph{typesetting} it: the latter is covered in the
% next section.
%
% When grabbing a \texttt{v}-type argument, \LaTeX{} first uses the kernel
% command \cs{dospecials} to turn off the \enquote{special} nature of characters.
% It then makes both spaces and tabs \enquote{active}, so that they can be given
% a custom definition. Any other characters are grabbed as-is: this means that if
% any characters have been made \enquote{special} and are not listed in
% \cs{dospecials}, an error will arise (see below).
%
% The characters that are grabbed as the argument are all those between two
% identical: in contrast to \cs{verb}, the characters \texttt{\textbackslash},
% |{|, |}| and |%| \emph{cannot} be used as the delimiter character. If any of
% the grabbed tokens have \enquote{special} meaning, an error will be issued.
%
% For the \texttt{+v}-type argument, which allows line breaks within the
% argument, newline characters are converted into \cs{obeyedline} commands. The
% standard definition of \cs{obeyedline} is simple |\par|, thus allowing the
% grabbed tokens to be used directly in typesetting. A local redefinition of
% \cs{obeyedline} can be used to achieve other outputs. For example, to retain
% blank lines whilst typesetting, one could use
% \begin{Verbatim}[gobble = 3]
%    \renewcommand*\obeyedline{\mbox{}\par}
% \end{Verbatim}
% More information about using these arguments in typesetting is in the following
% subsection.
%
% Some additional details that may be useful for those with more \TeX{}
% knowledge: do not worry if this does not make sense to you! Spaces and tabs are
% stored as active characters. In $8$-bit engines, non-ASCII characters are
% \enquote{active}, whilst other than the letters a--zA--Z, ASCII characters are
% \enquote{other}. In Unicode engines, non-ASCII codepoints will be either
% letters or \enquote{other}, based on the standard \LaTeX{} settings derived
% from Unicode data. For token-based comparisons, it is likely that the active
% spaces and tabs should be replaced: this can be done conveniently by expansion.
%
% \subsection{Typesetting verbatim-like material}
%
% In contrast to \cs{verb}, the \texttt{(+)v}-type argument is only about
% \emph{grabbing} the argument, not \emph{typesetting} it. As such, features that
% users often associate with \enquote{verbatim} are not automatically activated,
% e.g., selecting a monospaced font. Material grabbed by the \texttt{v}-type
% argument does not automatically suppress ligatures: with modern \TeX{} engines,
% this largely can be done without the token manipulation which \cs{verb} uses.
% (In \cs{verb}, ligatures are suppressed by making characters active and
% inserting a zero-width kern before the character itself.)
%
% The \cs{verb} command also selects a monospaced font: this is not intrinsic to
% verbatim material, so will need to be set up using for example \cs{ttfamily}.
% Similarly, the \texttt{verbatim} environment sets up the meaning of \cs{par}
% suitable for breaking lines.
%
% \subsection{Verbatim environments}
% \label{sec:cmd:verbenv}
%
% In some cases, when grabbing the body of an environment you will want the
% contents to be treated verbatim. This is available using the argument
% specification~\texttt{c}. Like the \texttt{b} specification, this has to be the
% last one. Thus for example
% \begin{Verbatim}[gobble = 3]
%    \NewDocumentEnvironment{MyVerbatim}{!O{\ttfamily} c}
%      {\begin{center} #1 #2\end{center}} {}
%    \begin{MyVerbatim}[\ttfamily\itshape]
%      % Some code is shown here
%      $y = mx + c$
%    \end{MyVerbatim}
% \end{Verbatim}
% will typeset verbatim the content, thus:
% \NewDocumentEnvironment{MyVerbatim}{!O{\ttfamily} c}
%   {\begin{center} #1 #2\end{center}} {}
% \begin{MyVerbatim}[\ttfamily\itshape]
%   % Some code is shown here
%   $y = mx + c$
% \end{MyVerbatim}
%
% Since grabbing the entire contents verbatim will result in there being no
% \cs{par} tokens, newlines are always permitted: there is no need for a
% |+|~modifier here. As for the \texttt{v} specification, newlines are stored as
% \cs{obeyedline}. In a similar fashion to the \texttt{b}~specification, by
% default \emph{newlines} are trimmed at both ends of the body. Putting the
% prefix |!| before \texttt{c} suppresses this trimming.
%
% Collection of the body takes place on a line-by-line basis: content is
% collected up to the end-of-line in the source, then examined before storage.
% This means that the line ending the environment (containing in the example
% above |\end{MyVerbatim}|) cannot have any text \emph{after} the end of the
% environment. Text \emph{before} the end of environment is treated normally,
% but note that there is no trailing \cs{obeyedline} added if there is text here.
% Other than optional arguments, no text is allowed on the opening line of the
% environment.
%
% Special handling is applied to a \texttt{o}, \texttt{O}, \texttt{d} or
% \texttt{D}~specification argument immediately before an
% \texttt{c}~specification. This means that when the optional argument is absent,
% the first character of the next line will be read with the correctly applied
% verbatim category code. Issues may arise if \emph{multiple} optional arguments
% are used before a \texttt{c}~specification: this will only work reliably where
% the optional tokens are \enquote{other} characters.
%
% For technical reasons, we recommend that spaces are \emph{not} ignored when
% searching for an optional argument before an \texttt{c} specification: this can
% be achieved by adding the \texttt{!} modifier as shown in the example.
% However, this is left as a choice for the user.
%
% \subsection{Performance}
%
% For document commands where the argument specification is entirely
% comprised of |m| or |+m| entries (or is entirely empty), the internal structure
% created by \cs{NewDocumentCommand} is essentially as efficient 
% as provided by |\newcommand(*)|. As such, document commands may replace
% constructs arising from \cs{newcommand}, etc., without a need to be concerned
% about performance. It should be noted that \cs{newcommand(*)} produces expandable
% results, so the direct replacement is \cs{NewExpandableDocumentCommand};
% in most cases, however, it is better to use \cs{NewDocumentCommand} to
% give more robust structures.
%
% \subsection{Details about argument delimiters}
%
% In normal (non-expandable) commands, the delimited types look for the
% initial delimiter by peeking ahead (using \pkg{expl3}'s |\peek_...|
% functions) looking for the delimiter token.  The token has to have the
% same meaning and \enquote{shape} of the token defined as delimiter.
% There are three possible cases of delimiters: character tokens, control
% sequence tokens, and active character tokens.  For all practical purposes
% of this description, active character tokens will behave exactly as
% control sequence tokens.
%
% \subsubsection{Character tokens}
%
% A character token is characterized by its character code, and its meaning
% is the category code~(|\catcode|).  When a command is defined, the meaning
% of the character token is fixed into the definition of the command and
% cannot change.  A command will correctly see an argument delimiter if
% the open delimiter has the same character and category codes as at the
% time of the definition.  For example in:
% \begin{Verbatim}[gobble = 3]
%    \NewDocumentCommand { \foobar } { D<>{default} } {(#1)}
%    \foobar <hello> \par
%    \char_set_catcode_letter:N <
%    \foobar <hello>
% \end{Verbatim}
% the output would be:
% \begin{Verbatim}[gobble = 3]
%    (hello)
%    (default)<hello>
% \end{Verbatim}
% as the open-delimiter |<| changed in meaning between the two calls to
% |\foobar|, so the second one doesn't see the |<| as a valid delimiter.
% Commands assume that if a valid open-delimiter was found, a matching
% close-delimiter will also be there.  If it is not (either by being
% omitted or by changing in meaning), a low-level \TeX{} error is raised
% and the command call is aborted.
%
% \subsubsection{Control sequence tokens}
%
% A control sequence (or control character) token is characterized by
% its name, and its meaning is its definition.
% A token cannot have two different meanings at the same time.
% When a control sequence is defined as delimiter in a command,
% it will be detected as delimiter whenever the control sequence name
% is found in the document regardless of its current definition.
% For example in:
% \begin{Verbatim}[gobble = 3]
%    \cs_set:Npn \x { abc }
%    \NewDocumentCommand { \foobar } { D\x\y{default} } {(#1)}
%    \foobar \x hello\y \par
%    \cs_set:Npn \x { def }
%    \foobar \x hello\y
% \end{Verbatim}
% the output would be:
% \begin{Verbatim}[gobble = 3]
%    (hello)
%    (hello)
% \end{Verbatim}
% with both calls to the command seeing the delimiter |\x|.
%
% \subsection{Creating new argument processors}
%
% \begin{variable}{\ProcessedArgument}
% \end{variable}
% Argument processors allow manipulation of a grabbed argument before it is
% passed to the underlying code. New processor implementations may be created
% as functions which take one trailing argument, and which leave their result in
% the \cs{ProcessedArgument} variable. For example, \cs{ReverseBoolean} is
% defined as
% \begin{Verbatim}[gobble = 3]
%    \ExplSyntaxOn
%    \cs_new_protected:Npn \ReverseBoolean #1
%      {
%        \bool_if:NTF #1
%          { \tl_set:Nn \ProcessedArgument { \c_false_bool } }
%          { \tl_set:Nn \ProcessedArgument { \c_true_bool } }
%      }
%    \ExplSyntaxOff
% \end{Verbatim}
% [As an aside: the code is written in \pkg{expl3}, so we don't have to
%   worry about spaces creeping into the definition.]
%
% \MaybeStop{}
%
%    \begin{macrocode}
%<@@=cmd>
%    \end{macrocode}
%
%    \begin{macrocode}
%<*2ekernel>
\message{document commands,}
%</2ekernel>
%    \end{macrocode}
%
% \changes{v1.0b}{2021/03/18}{Use \cs{NewModuleRelease}.}
% \changes{v1.0e}{2021/05/24}{Use \cs{msg_...} instead of \cs{__kernel_msg...}}
% \changes{v1.3k}{2026-03-12}{Switch from \texttt{x}- to \texttt{e}-type
%   expansion in expl3 code}
%
% \pkg{ltcmd} code contains an |^^@| character, which usually has
% catcode~15, so \cs{IncludeInRelease} will break when this code is
% being skipped, so we'll save the catcode of |^^@| to restore later:
%    \begin{macrocode}
%<*2ekernel|latexrelease>
%<latexrelease>\edef\@latexrelease@catcode@null{\the\catcode`\^^@ }
%<latexrelease>\catcode`\^^@=12
\ExplSyntaxOn
%<latexrelease>\NewModuleRelease{2020/10/01}{ltcmd}
%<latexrelease>                 {Document~command~parser}%
%    \end{macrocode}
%
%    \begin{macrocode}
\cs_generate_variant:Nn \tl_if_in:nnT { ee }
%    \end{macrocode}
%
% \subsection{Variables and constants}
%
% \begin{variable}{\l_@@_arg_spec_tl}
%   Holds the argument specification after normalization of shorthands.
%    \begin{macrocode}
\tl_new:N \l_@@_arg_spec_tl
%    \end{macrocode}
% \end{variable}
%
% \begin{variable}{\l_@@_args_tl}
%   Token list variable for grabbed arguments.
%    \begin{macrocode}
\tl_new:N \l_@@_args_tl
%    \end{macrocode}
% \end{variable}
%
% \begin{variable}{\l_@@_args_i_tl, \l_@@_args_ii_tl}
%   Hold the modified arguments when dealing with default values or
%   processors.
%    \begin{macrocode}
\tl_new:N \l_@@_args_i_tl
\tl_new:N \l_@@_args_ii_tl
%    \end{macrocode}
% \end{variable}
%
% \begin{variable}{\l_@@_current_arg_int}
%   The number of the current argument being set up: this is used to
%   make sure there are at most 9 arguments, then for creating the
%   expandable auxiliary functions and knowing how many arguments the
%   code function should take.
%    \begin{macrocode}
\int_new:N \l_@@_current_arg_int
%    \end{macrocode}
% \end{variable}
%
% \begin{variable}{\l_@@_total_args_int}
% \changes{v1.3a}{2025-01-23}{New variable}
%   The total number of arguments found during normalization: this is required
%   where special action is needed for the penultimate argument.
%    \begin{macrocode}
\int_new:N \l_@@_total_args_int
%    \end{macrocode}
% \end{variable}
%
% \begin{variable}{\l_@@_last_mandatory_arg_int}
% \changes{v1.3c}{2025-04-12}{New variable}
% \changes{v1.3f}{2025-09-23}{Rename variable}
%   Needed to allow handling of verbatim catcodes at the end of the
%   setup.
%    \begin{macrocode}
\int_new:N \l_@@_last_mandatory_arg_int
%    \end{macrocode}
% \end{variable}
%
% \begin{variable}{\l_@@_defaults_bool, \l_@@_defaults_tl}
%   The boolean indicates whether there are any argument with default
%   value other than \cs{NoValue}; the token list holds the code to
%   determine these default values in terms of other arguments.
%    \begin{macrocode}
\bool_new:N \l_@@_defaults_bool
\tl_new:N \l_@@_defaults_tl
%    \end{macrocode}
% \end{variable}
%
% \begin{variable}{\l_@@_environment_bool}
%   Generating environments uses the same mechanism as generating functions.
%   However, full processing of arguments is always needed for environments,
%   and so the function-generating code needs to know this.
%   This variable is also used at run time to give correct error messages.
%    \begin{macrocode}
\bool_new:N \l_@@_environment_bool
%    \end{macrocode}
% \end{variable}
%
% \begin{variable}{\l_@@_environment_str}
%   Name of the environment, used at definition time and at run time.
%    \begin{macrocode}
\str_new:N \l_@@_environment_str
%    \end{macrocode}
% \end{variable}
%
% \begin{variable}{\l_@@_expandable_bool}
%   Used to indicate if an expandable command is begin generated, as this
%   affects both the acceptable argument types and how they are implemented.
%    \begin{macrocode}
\bool_new:N \l_@@_expandable_bool
%    \end{macrocode}
% \end{variable}
%
% \begin{variable}{\l_@@_expandable_aux_name_tl}
%   Used to create pretty-printing names for the auxiliaries: although the
%   immediate definition does not vary, the full expansion does and so it
%   does not count as a constant.
%    \begin{macrocode}
\tl_new:N \l_@@_expandable_aux_name_tl
\tl_set:Nn \l_@@_expandable_aux_name_tl
  {
    \l_@@_function_tl \c_space_tl
    ( arg~ \int_use:N \l_@@_current_arg_int )
  }
%    \end{macrocode}
% \end{variable}
%
% \begin{variable}{\g_@@_grabber_int}
%   Used (in exceptional cases) to get unique names for grabbers used by
%   expandable commands.
%    \begin{macrocode}
\int_new:N \g_@@_grabber_int
%    \end{macrocode}
% \end{variable}
%
% \begin{variable}{\l_@@_fn_tl}
%   For passing the pre-formed name of the auxiliary to be used as the
%   parsing function.
%    \begin{macrocode}
\tl_new:N \l_@@_fn_tl
%    \end{macrocode}
% \end{variable}
%
% \begin{variable}{\l_@@_fn_code_tl}
%   For passing the pre-formed name of the auxiliary that contains the
%   actual code.
%    \begin{macrocode}
\tl_new:N \l_@@_fn_code_tl
%    \end{macrocode}
% \end{variable}
%
% \begin{variable}{\l_@@_function_tl}
%   Holds the control sequence name of the function currently being
%   defined: used to avoid passing this as an argument and to avoid repeated
%   use of \cs{cs_to_str:N}.
%    \begin{macrocode}
\tl_new:N \l_@@_function_tl
%    \end{macrocode}
% \end{variable}
%
% \begin{variable}{\l_@@_grab_expandably_bool}
%   When defining a non-expandable command, indicates whether the
%   arguments can all safely be grabbed by expandable grabbers.  This is
%   to support abuses of \pkg{xparse} that use protected functions
%   inside csname constructions.
%    \begin{macrocode}
\bool_new:N \l_@@_grab_expandably_bool
%    \end{macrocode}
% \end{variable}
%
% \begin{variable}{\l_@@_obey_spaces_bool}
%   For trailing optionals.
%    \begin{macrocode}
\bool_new:N \l_@@_obey_spaces_bool
%    \end{macrocode}
% \end{variable}
%
% \begin{variable}{\l_@@_last_delimiters_tl}
%   Holds the delimiters (first tokens) of all optional arguments since
%   the previous mandatory argument, to warn about cases where it would
%   be impossible to omit optional arguments completely because the
%   following mandatory argument has the same delimiter as one of the
%   optional arguments.
%    \begin{macrocode}
\tl_new:N \l_@@_last_delimiters_tl
%    \end{macrocode}
% \end{variable}
%
% \begin{variable}{\l_@@_long_bool}
%   Used to indicate that an argument is long, on a per-argument basis.
%    \begin{macrocode}
\bool_new:N \l_@@_long_bool
%    \end{macrocode}
% \end{variable}
%
% \begin{variable}{\l_@@_suppress_strip_bool}
% \changes{v1.1a}{2022/08/10}{New switch}
%   Used to indicate that an a pair of braces should not be stripped from
%   an optional argument.
%    \begin{macrocode}
\bool_new:N \l_@@_suppress_strip_bool
%    \end{macrocode}
% \end{variable}
%
% \begin{variable}{\l_@@_m_args_int}
%   The number of \texttt{m} arguments: if this is the same as the total
%   number of arguments, then a short-cut can be taken in the creation of
%   the grabber code.
%    \begin{macrocode}
\int_new:N \l_@@_m_args_int
%    \end{macrocode}
% \end{variable}
%
% \begin{variable}{\l_@@_prefixed_bool}
%   When preparing the signature of non-expandable commands, indicates
%   that the current argument is affected by a processor or by |+|
%   (namely is long).
%    \begin{macrocode}
\bool_new:N \l_@@_prefixed_bool
%    \end{macrocode}
% \end{variable}
%
% \begin{variable}{\l_@@_process_all_tl, \l_@@_process_one_tl, \l_@@_process_some_bool}
%   When preparing the signature, the processors that will be applied to
%   a given argument are collected in \cs{l_@@_process_one_tl}, while
%   \cs{l_@@_process_all_tl} contains processors for all arguments.  The
%   boolean indicates whether there are any processors (to bypass the
%   whole endeavour otherwise).
%    \begin{macrocode}
\tl_new:N \l_@@_process_all_tl
\tl_new:N \l_@@_process_one_tl
\bool_new:N \l_@@_process_some_bool
%    \end{macrocode}
% \end{variable}
%
% \begin{variable}{\l_@@_saved_args_tl}
%   Stores \cs{l_@@_args_tl} to deal with space-trimming of
%   \texttt{b}-type arguments.
%    \begin{macrocode}
\tl_new:N \l_@@_saved_args_tl
%    \end{macrocode}
% \end{variable}
%
% \begin{variable}{\l_@@_signature_tl}
%   Used when constructing the signature (code for argument grabbing) to
%   hold what will become the implementation of the main function.
%   When arguments are grabbed (at point of use of the command/environment),
%   it also stores the code for grabbing the remaining arguments.
%    \begin{macrocode}
\tl_new:N \l_@@_signature_tl
%    \end{macrocode}
% \end{variable}
%
% \begin{variable}
%   {\l_@@_some_obey_spaces_bool, \l_@@_some_long_bool, \l_@@_some_short_bool}
%   These flags are set while normalizing the argument specification.
%   The \texttt{obey_spaces} one is used to detect when |!| is used on
%   an argument that is not a trailing optional argument.
%   The other two are used to check whether all short arguments appear
%   before long arguments: this is needed to grab arguments expandably.
%   As soon as the first long argument is seen (other than
%   \texttt{t}-type, whose long status is ignored) the
%   \texttt{some_long} flag is set.  The \texttt{some_short} flag is
%   used for expandable commands, to know whether to define a short
%   auxiliary too.
%    \begin{macrocode}
\bool_new:N \l_@@_some_obey_spaces_bool
\bool_new:N \l_@@_some_long_bool
\bool_new:N \l_@@_some_short_bool
%    \end{macrocode}
% \end{variable}
%
% \begin{variable}{\l_@@_final_verb_bool}
% \changes{v1.3a}{2025-01-23}{New variable}
%   Needed to establish whether optional arguments should be collected
%   \enquote{verbatim safe}.
%    \begin{macrocode}
\bool_new:N \l_@@_final_verb_bool
%    \end{macrocode}
% \end{variable}
%
% \begin{variable}{\q_@@_recursion_tail,\q_@@_recursion_stop}
% \begin{macro}{\@@_if_recursion_tail_stop_do:Nn}
% \begin{macro}{\@@_use_i_delimit_by_q_recursion_stop:nw}
%   Quarks and functions for internal processing.
%    \begin{macrocode}
\quark_new:N \q_@@_recursion_tail
\quark_new:N \q_@@_recursion_stop
\__kernel_quark_new_test:N \@@_if_recursion_tail_stop_do:Nn
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \end{variable}
%
% \begin{variable}{\l_@@_tmp_prop, \l_@@_tmpa_tl, \l_@@_tmpb_tl}
% \begin{macro}{\@@_tmp:w}
%   Scratch space.
%    \begin{macrocode}
\prop_new:N \l_@@_tmp_prop
\tl_new:N \l_@@_tmpa_tl
\tl_new:N \l_@@_tmpb_tl
\cs_new_eq:NN \@@_tmp:w ?
%    \end{macrocode}
% \end{macro}
% \end{variable}
%
%   With \pkg{xparse}, information about commands being (re)defined was
%   switched off by default, unless the |log-declarations| package
%   option was used, so here we'll switch that off as well.
%    \begin{macrocode}
\msg_redirect_module:nnn { cmd } { info } { none }
%    \end{macrocode}
%
%   Also add \pkg{cmd} to the \pkg{LaTeX} messages.
%    \begin{macrocode}
\prop_gput:Nnn \g_msg_module_type_prop { cmd } { LaTeX }
%    \end{macrocode}
%
% \subsection{Declaring commands and environments}
%
% \begin{macro}{\@@_declare_cmd:Nnn, \@@_declare_expandable_cmd:Nnn}
% \begin{macro}{\@@_declare_cmd_aux:Nnn}
% \begin{macro}{\@@_declare_cmd_internal:Nnnn}
%   The main functions for creating commands set the appropriate flag then
%   use the same internal code to do the definition.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_declare_cmd:Nnn
  {
    \bool_set_false:N \l_@@_expandable_bool
    \@@_declare_cmd_aux:Nnn
  }
\cs_new_protected:Npn \@@_declare_expandable_cmd:Nnn
  {
    \bool_set_true:N \l_@@_expandable_bool
    \@@_declare_cmd_aux:Nnn
  }
%    \end{macrocode}
%  The first stage is to log information, both for the user in the log and
%  for programmatic use in a property list of all declared commands.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_declare_cmd_aux:Nnn #1#2#3
  {
    \cs_if_exist:NTF #1
      {
        \msg_info:nnee { cmd } { redefine }
          { \token_to_str:N #1 } { \tl_to_str:n {#2} }
      }
      {
        \bool_lazy_or:nnT
          { \cs_if_exist_p:c { \cs_to_str:N #1 ~ code } }
          { \cs_if_exist_p:c { \cs_to_str:N #1 ~ defaults } }
          {
            \msg_warning:nne { cmd } { unsupported-let }
              { \token_to_str:N #1 }
          }
        \msg_info:nnee { cmd } { define-command }
          { \token_to_str:N #1 } { \tl_to_str:n {#2} }
      }
    \bool_set_false:N \l_@@_environment_bool
    \@@_declare_cmd_internal:Nnnn #1 {#2} {#3} { }
  }
%    \end{macrocode}
%   At definition time, the variable \cs{l_@@_fn_tl} is only used for error messages.
%   The real business of defining a document command starts with setting up
%   the appropriate name, then normalizing the argument specification to get rid of
%   shorthands.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_declare_cmd_internal:Nnnn #1#2#3#4
  {
    \tl_set:Ne \l_@@_function_tl { \cs_to_str:N #1 }
    \@@_normalize_arg_spec:n {#2}
    \exp_args:No \@@_prepare_signature:n \l_@@_arg_spec_tl
    \@@_declare_cmd_code:Nnn #1 {#2} {#3}
    #4
    \@@_break_point:n {#2}
  }
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \end{macro}
%
% \begin{macro}{\@@_break_point:n}
%   A marker used to escape from creating a definition if necessary.
%    \begin{macrocode}
\cs_new_eq:NN \@@_break_point:n \use_none:n
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}[EXP]{\@@_all_m_check:n, \@@_all_m_check_aux:n}
%   A quick loop to check for all |(+)m|-type arguments.
%    \begin{macrocode}
\cs_new:Npn \@@_all_m_check:n #1
  { \tl_map_function:nN {#1} \@@_all_m_check_aux:n }
\cs_new:Npn \@@_all_m_check_aux:n #1
  {
    \str_if_eq:nnF {#1} { m }
      {
        \str_if_eq:nnF {#1} { + }
          { X }
      }
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\@@_declare_cmd_code:Nnn}
% \begin{macro}
%   {
%     \@@_declare_cmd_optimized:Nnn,
%     \@@_declare_cmd_code_aux:Nnn,
%     \@@_declare_cmd_code_expandable:Nnn
%   }
% \begin{macro}{\@@_start_optimized:}
% \changes{v1.2b}{2023/12/01}
%   {Optimize cmd creation for all-\texttt{m} arguments}
%   At this stage we can check for a short-cut possibility: if the argument
%   specification is made up of just |(+)m| tokens, and if all arguments are
%   either short or long, then we can produce an optimized document command.
%   This only applies to document commands, not creation of environments (which
%   are more complex).
%    \begin{macrocode}
\cs_new_protected:Npn \@@_declare_cmd_code:Nnn #1#2
  {
    \bool_lazy_any:nTF
      {
        { \l_@@_environment_bool }
        {
          \bool_lazy_and_p:nn
            { \l_@@_some_short_bool }
            { \l_@@_some_long_bool }
        }
        { ! \tl_if_blank_p:e { \@@_all_m_check:n {#2} } }
      }
      {
        \tl_set:Ne \l_@@_fn_tl
          { \exp_not:c { \l_@@_function_tl \c_space_tl } }
        \bool_if:NTF \l_@@_grab_expandably_bool
          { \@@_declare_cmd_code_expandable:Nnn }
          { \@@_declare_cmd_code_aux:Nnn }
      }
      { \@@_declare_cmd_optimized:Nnn }
        #1 {#2}
   }
%    \end{macrocode}
%   The optimized version of commands just has to worry about whether to make
%   them protected or long. The commands start with an expandable marker so
%   that other parts of the kernel know these are set up by \pkg{ltcmd}.
%   We need the two layers of redirection so that the \texttt{code} internal
%   function has the same form as it would for any other document command.
%   Optimization means that there is no \cs{group_align_safe_begin:} before
%   grabbing the arguments, so anything involving |&| tokens will not work.
%   However, this is really only intended for making optional argument
%   processing safe anyway, so in practice should not be an issue.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_declare_cmd_optimized:Nnn #1#2#3
  {
    \bool_if:NTF \l_@@_expandable_bool
      { \cs_set_nopar:Npe }
      { \cs_set_protected_nopar:Npe }
        #1
        {
          \exp_not:N \@@_start_optimized:
          \exp_not:c { \l_@@_function_tl \c_space_tl code }
        }
    \exp_args:Ncc \cs_generate_from_arg_count:NNnn
      { \l_@@_function_tl \c_space_tl code }
      {
        cs_set
        \bool_if:NF \l_@@_expandable_bool { _protected }
        \bool_if:NF \l_@@_some_long_bool { _nopar }
        :Npn
      }
      \l_@@_current_arg_int
      {#3}
  }
\cs_new:Npn \@@_start_optimized: { }
%    \end{macrocode}
%   Standard functions call \cs{@@_start:nNNnnn}, which receives the
%   argument specification, an auxiliary used for
%   grabbing arguments, an auxiliary containing the code, and then the
%   signature, default arguments, and processors.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_declare_cmd_code_aux:Nnn #1#2#3
  {
    \cs_generate_from_arg_count:cNnn
      { \l_@@_function_tl \c_space_tl code }
      \cs_set_protected:Npn \l_@@_current_arg_int {#3}
    \cs_set_protected_nopar:Npe #1
      {
        \bool_if:NTF \l_@@_environment_bool
          {
            \@@_start_env:nnnnn { \exp_not:n {#2} }
              { \l_@@_environment_str }
          }
          {
            \@@_start:nNNnnn { \exp_not:n {#2} }
              \exp_not:c { \l_@@_function_tl \c_space_tl }
              \exp_not:c { \l_@@_function_tl \c_space_tl code }
          }
          { \exp_not:o \l_@@_signature_tl }
          {
            \bool_if:NT \l_@@_defaults_bool
              { \exp_not:o \l_@@_defaults_tl }
          }
          {
            \bool_if:NT \l_@@_process_some_bool
              { \exp_not:o \l_@@_process_all_tl }
          }
      }
  }
%    \end{macrocode}
%   Expandable functions and functions whose arguments can be grabbed
%   expandably call \cs{@@_start_expandable:nNNNNn}, which receives the
%   argument specification, four auxiliaries (two for grabbing arguments, one for
%   the code, and one for default arguments), and finally the signature.
%   Non-expandable functions that take this branch should nevertheless
%   be protected, as well as their \texttt{code} function.  They will
%   only be expanded in contexts such as constructing a csname.
%   The two grabbers (named after the function with one or two spaces)
%   are needed when there are both short and long arguments; otherwise
%   the same grabber is included twice in the definition.  If all
%   arguments are long or all are short the (only) grabber is defined
%   correspondingly to be long/short.  Otherwise two grabbers are
%   defined, one long, one short.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_declare_cmd_code_expandable:Nnn #1#2#3
  {
    \exp_args:Ncc \cs_generate_from_arg_count:NNnn
      { \l_@@_function_tl \c_space_tl code }
      { cs_set \bool_if:NF \l_@@_expandable_bool { _protected } :Npn }
      \l_@@_current_arg_int {#3}
    \bool_if:NT \l_@@_defaults_bool
      {
        \use:e
          {
            \cs_generate_from_arg_count:cNnn
              { \l_@@_function_tl \c_space_tl defaults }
              \cs_set:Npn \l_@@_current_arg_int
              { \exp_not:o \l_@@_defaults_tl }
          }
      }
    \bool_if:NTF \l_@@_expandable_bool
      { \cs_set_nopar:Npe } { \cs_set_protected_nopar:Npe } #1
      {
        \exp_not:N \@@_start_expandable:nNNNNn
          { \exp_not:n {#2} }
          \exp_not:c { \l_@@_function_tl \c_space_tl }
          \exp_not:c
            {
              \l_@@_function_tl \c_space_tl
              \bool_if:NT \l_@@_some_short_bool
                { \bool_if:NT \l_@@_some_long_bool { \c_space_tl } }
            }
          \exp_not:c { \l_@@_function_tl \c_space_tl code }
          \bool_if:NTF \l_@@_defaults_bool
            { \exp_not:c { \l_@@_function_tl \c_space_tl defaults } }
            { ? }
          { \exp_not:o \l_@@_signature_tl }
      }
    \bool_if:NTF \l_@@_some_long_bool
      {
        \bool_if:NT \l_@@_some_short_bool
          {
            \cs_set_nopar:cpe { \l_@@_function_tl \c_space_tl \c_space_tl }
              ##1##2 { ##1 {##2} }
          }
        \cs_set:cpe
      }
      { \cs_set_nopar:cpe }
          { \l_@@_function_tl \c_space_tl } ##1##2 { ##1 {##2} }
  }
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \end{macro}
%
% \begin{macro}{\@@_declare_env:nnnn, \@@_declare_env:ennn}
% \changes{v1.2f}{2024/07/06}{Use space-trimmed envname directly (gh/1399)}
% \begin{macro}{\@@_declare_env_internal:nnnn}
% \begin{macro}{\@@_set_environment_end:n}
%   The lead-off to creating an environment is much the same as that for
%   creating a command: issue the appropriate message, store the argument
%   specification then hand off to an internal function.
%    \begin{macrocode}
%<latexrelease>\IncludeInRelease{2024/11/01}{\@@_declare_env:nnnn}%
%<latexrelease>                 {Use~space-trimmed~envname~directly}
\cs_new_protected:Npn \@@_declare_env:nnnn #1#2
  {
    \str_set:Nn \l_@@_environment_str {#1}
    \cs_if_exist:cTF { #1 }
      { \msg_info:nnnn { cmd } { redefine-env } { #1 } { #2 } }
      { \msg_info:nnnn { cmd } { define-env } { #1 } { #2 } }
    \bool_set_false:N \l_@@_expandable_bool
    \bool_set_true:N \l_@@_environment_bool
    \@@_declare_env_internal:nnnn {#1} {#2}
  }
\cs_generate_variant:Nn \@@_declare_env:nnnn { e }
%<latexrelease>\EndIncludeInRelease
%<latexrelease>\IncludeInRelease{2024/06/01}{\@@_declare_env:nnnn}%
%<latexrelease>                 {Use~space-trimmed~envname~directly}
%<latexrelease>\cs_new_protected:Npn \@@_declare_env:nnnn #1#2
%<latexrelease>  {
%<latexrelease>    \str_set:Nx \l_@@_environment_str {#1}
%<latexrelease>    \str_set:Nx \l_@@_environment_str
%<latexrelease>      { \tl_trim_spaces:o { \l_@@_environment_str } }
%<latexrelease>    \cs_if_exist:cTF { \l_@@_environment_str }
%<latexrelease>      {
%<latexrelease>        \msg_info:nnxx { cmd } { redefine-env }
%<latexrelease>          { \l_@@_environment_str } { \tl_to_str:n {#2} }
%<latexrelease>      }
%<latexrelease>      {
%<latexrelease>        \msg_info:nnxx { cmd } { define-env }
%<latexrelease>          { \l_@@_environment_str } { \tl_to_str:n {#2} }
%<latexrelease>      }
%<latexrelease>    \bool_set_false:N \l_@@_expandable_bool
%<latexrelease>    \bool_set_true:N \l_@@_environment_bool
%<latexrelease>    \exp_args:NV \@@_declare_env_internal:nnnn
%<latexrelease>      \l_@@_environment_str {#2}
%<latexrelease>  }
%<latexrelease>
%<latexrelease>\EndIncludeInRelease
%    \end{macrocode}
%   Creating a document environment requires a few more steps than creating
%   a single command. In order to pass the arguments of the command to the
%   end of the function, it is necessary to store the grabbed arguments.
%   To do that, the function used at the end of the environment has to be
%   redefined to contain the appropriate information. To minimize the amount
%   of expansion at point of use, the code here is expanded now as well as
%   when used.
%   The last argument of \cs{@@_declare_cmd_internal:Nnnn} is only run
%   if the definition succeeded.  In package mode this ensures that the
%   original definition of the environment is not changed if the
%   definition fails for any reason.  This also avoids an error when
%   defining the \verb*|end aux | function when the user asks for more
%   than $9$ arguments.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_declare_env_internal:nnnn #1#2#3#4
  {
    \exp_args:Nc \@@_declare_cmd_internal:Nnnn { environment~ #1 } {#2}
      {#3}
      {
        \cs_set_nopar:cpe { environment~ #1 ~end }
          { \exp_not:c { environment~ #1 ~end~aux } }
        \cs_generate_from_arg_count:cNnn
          { environment~ #1 ~end~aux~ } \cs_set:Npn
          \l_@@_current_arg_int {#4}
        \cs_set_eq:cc {#1}       { environment~ #1 }
        \cs_set_eq:cc { end #1 } { environment~ #1 ~end }
      }
  }
\cs_new_protected:Npn \@@_set_environment_end:n #1
  {
    \cs_set_nopar:cpe { environment~ #1 ~end~aux }
      {
        \exp_not:c { environment~ #1 ~end~aux~ }
        \exp_not:o \l_@@_args_tl
      }
  }
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \end{macro}
%
% \subsection{Structure of \pkg{xparse} commands}
%
% \begin{macro}{\@@_start_env:nnnnn, \@@_start:nNNnnn}
%   For error messages that occur during run-time when getting arguments
%   of environments it is necessary to keep track of the environment
%   name.  We begin non-expandable commands with a token equal to
%   \cs{scan_stop:}, whose name gives a reasonable error message if the
%   command is used inside a csname and protects against
%   \texttt{f}-expansion.  This is useless for environments since
%   \cs{begin} is already not expandable.  Both the command and
%   environment codes start with \cs{group_align_safe_begin:}, then
%   \cs{@@_run_code:} (used by both) does \cs{group_align_safe_end:}, so
%   that delimited arguments may be grabbed in alignments if they
%   contain and alignment tab token (see latex3/latex3/issues/839).
%    \begin{macrocode}
\cs_new_protected:Npn \@@_start_env:nnnnn #1#2
  {
    \conditionally@traceoff
    \group_align_safe_begin:
    \str_set:Nn \l_@@_environment_str {#2}
    \bool_set_true:N \l_@@_environment_bool
    \@@_start_aux:ccnnnn
      { environment~ \l_@@_environment_str \c_space_tl }
      { environment~ \l_@@_environment_str \c_space_tl code }
      {#1}
  }
\cs_new_protected:Npe \@@_start:nNNnnn #1#2#3
  {
    \exp_not:c { xparse~function~is~not~expandable }
    \exp_not:N \conditionally@traceoff
    \exp_not:N \group_align_safe_begin:
    \exp_not:n { \bool_set_false:N \l_@@_environment_bool }
    \exp_not:N \@@_start_aux:NNnnnn
    #2 #3 {#1}
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\@@_start_aux:NNnnnn, \@@_start_aux:ccnnnn}
% \changes{v1.0i}{2021/12/02}
%    {Correct defaults for optional arguments in end-of-environment code (gh/712)}
%   This sets up a few variables to minimize the boilerplate code
%   included in all \pkg{xparse}-defined commands.  It then runs the
%   grabbers~|#4|.  Again, the argument specification |#1| is only for
%   diagnostics.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_start_aux:NNnnnn #1#2#3#4#5#6
  {
    \tl_clear:N \l_@@_args_tl
    \tl_set:Nn \l_@@_fn_tl {#1}
    \tl_set:Nn \l_@@_fn_code_tl {#2}
    \tl_set:Nn \l_@@_defaults_tl {#5}
    \tl_set:Nn \l_@@_process_all_tl {#6}
    #4
    \@@_run_code:
  }
\cs_generate_variant:Nn \@@_start_aux:NNnnnn { cc }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\@@_run_code:}
% \changes{v1.0i}{2021/12/02}
%    {Correct defaults for optional arguments in end-of-environment code (gh/712)}
%   After arguments are grabbed, this function is responsible for
%   inserting default values, running processors, and finally doing
%   \cs{group_align_safe_end:} as promised, and running the code.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_run_code:
  {
    \tl_if_empty:NF \l_@@_defaults_tl { \@@_defaults: }
    \tl_if_empty:NF \l_@@_process_all_tl { \@@_args_process: }
    \bool_if:NT \l_@@_environment_bool
      { \exp_args:No \@@_set_environment_end:n \l_@@_environment_str }
    \group_align_safe_end:
    \conditionally@traceon
    \exp_after:wN \l_@@_fn_code_tl \l_@@_args_tl
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\@@_defaults:}
% \begin{macro}{\@@_defaults_def:, \@@_defaults_def:nn, \@@_defaults_def:nnn}
% \begin{macro}{\@@_defaults_aux:,  \@@_defaults_error:w}
%   First construct \cs{@@_tmp:w} (see below) that will receive
%   the arguments found so far and determine default values for any
%   missing argument.  Then call it repeatedly until the set of
%   arguments stabilizes.  Since that could lead to an infinite loop we
%   only call it up to nine times, the maximal number needed for
%   stabilization if there is a chain of arguments that depend on each
%   other.  If that fails to stabilize raise an error.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_defaults:
  {
    \@@_defaults_def:
    \tl_set_eq:NN \l_@@_args_i_tl \l_@@_args_tl
    \@@_defaults_aux: \@@_defaults_aux: \@@_defaults_aux:
    \@@_defaults_aux: \@@_defaults_aux: \@@_defaults_aux:
    \@@_defaults_aux: \@@_defaults_aux: \@@_defaults_aux:
    \@@_defaults_error:w
    \q_recursion_stop
    \tl_set_eq:NN \l_@@_args_tl \l_@@_args_i_tl
  }
\cs_new_protected:Npn \@@_defaults_aux:
  {
    \tl_set:Ne \l_@@_args_ii_tl
      { \exp_after:wN \@@_tmp:w \l_@@_args_i_tl }
    \tl_if_eq:NNT \l_@@_args_ii_tl \l_@@_args_i_tl
      { \use_none_delimit_by_q_recursion_stop:w }
    \tl_set_eq:NN \l_@@_args_i_tl \l_@@_args_ii_tl
  }
\cs_new_protected:Npn \@@_defaults_error:w \q_recursion_stop
  {
    \msg_error:nne { cmd } { default-loop }
      { \@@_environment_or_command: }
  }
%    \end{macrocode}
%
% \changes{v1.1e}{2023/05/26}
%         {Use simpler variant \cs{cs_generate_from_arg_count:NNno}}
%  \changes{v1.3k}{2026-03-13}{Tidy up variants}
%   To construct \cs{@@_tmp:w}, first go through the arguments
%   found and the corresponding defaults, building a token list with
%   |{#|\meta{arg number}|}| for arguments found in the input (whose
%   default will not be used) and otherwise
%   |{|\cs{exp_not:n}\Arg{default}|}| for arguments whose default will
%   be used.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_defaults_def:
  {
    \tl_clear:N \l_@@_tmpa_tl
    \int_zero:N \l_@@_current_arg_int
    \@@_tl_mapthread_function:NNN \l_@@_args_tl \l_@@_defaults_tl
      \@@_defaults_def:nn
    \cs_generate_from_arg_count:NNno \@@_tmp:w \cs_set:Npn
      \l_@@_current_arg_int \l_@@_tmpa_tl
  }
\cs_new_protected:Npn \@@_defaults_def:nn
  {
    \int_incr:N \l_@@_current_arg_int
    \exp_args:NV \@@_defaults_def:nnn \l_@@_current_arg_int
  }
\cs_new_protected:Npn \@@_defaults_def:nnn #1#2#3
  {
    \tl_put_right:Ne \l_@@_tmpa_tl
      {
        {
          \exp_not:N \exp_not:n
            {
              \tl_if_novalue:nTF {#2}
                { \exp_not:o {#3} }
                { \exp_not:n { ## #1 } }
            }
        }
      }
  }
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \end{macro}
%
% \begin{macro}{\@@_args_process:}
% \begin{macro}{\@@_args_process_loop:nn, \@@_args_process_aux:n}
%   Loop through arguments (stored in \cs{l_@@_args_tl}) and the
%   corresponding processors (in \cs{l_@@_process_all_tl})
%   simultaneously, apply all processors for each argument and store the
%   result back into \cs{l_@@_args_tl}.  To allow processors to depend
%   on other arguments, for every processor define a temporary auxiliary
%   that receives all arguments \cs{l_@@_args_tl}.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_args_process:
  {
    \tl_clear:N \l_@@_args_ii_tl
    \@@_tl_mapthread_function:NNN
      \l_@@_args_tl
      \l_@@_process_all_tl
      \@@_args_process_loop:nn
    \tl_set_eq:NN \l_@@_args_tl \l_@@_args_ii_tl
  }
\cs_new_protected:Npn \@@_args_process_loop:nn #1#2
  {
    \tl_set:Nn \ProcessedArgument {#1}
    \tl_if_novalue:nF {#1}
      { \tl_map_function:nN {#2} \@@_args_process_aux:n }
    \tl_put_right:No \l_@@_args_ii_tl
      { \exp_after:wN { \ProcessedArgument } }
  }
\cs_new_protected:Npn \@@_args_process_aux:n #1
  {
    \cs_generate_from_arg_count:NNnn \@@_tmp:w \cs_set:Npn
      { \tl_count:N \l_@@_args_tl } {#1}
    \exp_args:NNNo \exp_after:wN \@@_tmp:w \l_@@_args_tl
      { \ProcessedArgument }
  }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}[EXP]{\@@_start_expandable:nNNNNn}
%   This is called for all expandable commands.  |#6| is the signature,
%   responsible for grabbing arguments.  |#5| is used to determine
%   default values (or is |?| if there are none).  |#4| is the code to run.
%   |#2|~and~|#3| are functions (named after the command) that grab a single
%   argument in the input stream (|#3|~is~short).  The argument specification |#1| is
%   only used by diagnostic functions.  Same as for the non-expandable
%   version, this starts with \cs{group_align_safe_begin:}, which
%   expands to nothing, so may be safely used in an expandable context.
%    \begin{macrocode}
\cs_new:Npn \@@_start_expandable:nNNNNn #1#2#3#4#5#6
  {
    \group_align_safe_begin:
    #6 \@@_end_expandable:NNw #5 #4 \q_@@ #2#3
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}[EXP]{\@@_end_expandable:NNw}
% \begin{macro}[EXP]{\@@_end_expandable_aux:w}
% \begin{macro}[EXP]{\@@_end_expandable_aux:nNNNN}
% \begin{macro}[EXP]{\@@_end_expandable_defaults:nnnNNn}
% \begin{macro}[EXP]{\@@_end_expandable_defaults:nnw}
% \begin{macro}[EXP]{\@@_end_expandable_defaults:nw}
%   Followed by a function |#1| to determine default values (or |?| if
%   there are no defaults), the code
%   |#2|, arguments that have been grabbed, then \cs{q_@@} and two generic
%   grabbers.  The idea to find default values is similar to the
%   non-expandable case but we cannot define an auxiliary function, so
%   at every step in the loop we need to go through all arguments
%   searching for which ones started out as \cs{NoValue} and replacing
%   these by the newly computed values.  In fact we need to keep track
%   of three versions of all arguments: the original version, the
%   previous version with default values, and the currently built
%   version (first argument of \cs{@@_end_expandable_defaults:nnnNNn}).
%    \begin{macrocode}
\cs_new:Npn \@@_end_expandable:NNw #1#2
  { \@@_end_expandable_aux:w #1#2 \prg_do_nothing: }
\cs_new:Npn \@@_end_expandable_aux:w #1#2#3 \q_@@
  { \exp_args:No \@@_end_expandable_aux:nNNNN {#3} #1 #2 }
\cs_new:Npn \@@_end_expandable_aux:nNNNN #1#2#3#4#5
  {
    \token_if_eq_charcode:NNT ? #2 { \exp_after:wN \use_iv:nnnn }
    \@@_end_expandable_defaults:nnnNNn {#1} { } {#1} #2#3
      { } { } { } { } { } { } { } { } { } { }
      {
        \msg_expandable_error:nnf { cmd } { default-loop }
          { \exp_args:Nf \tl_trim_spaces:n { \token_to_str:N #4 } }
        \use_iv:nnnn
      }
    \q_stop
  }
\cs_new:Npn \@@_end_expandable_defaults:nnnNNn #1#2#3#4#5#6
  {
    #6
    \str_if_eq:nnTF {#1} {#2}
      { \use_i_delimit_by_q_stop:nw { \group_align_safe_end: #5 #1 } }
      {
        \exp_args:No \@@_tl_mapthread_function:nnN
          { #4 #1 } {#3}
          \@@_end_expandable_defaults:nnw
        \@@_end_expandable_defaults:nnnNNn { } {#1} {#3} #4 #5
      }
  }
\cs_new:Npn \@@_end_expandable_defaults:nnw #1#2
  {
    \tl_if_novalue:nTF {#2}
      { \exp_args:No \@@_end_expandable_defaults:nw {#1} }
      { \@@_end_expandable_defaults:nw {#2} }
  }
\cs_new:Npn \@@_end_expandable_defaults:nw
    #1#2 \@@_end_expandable_defaults:nnnNNn #3
  { #2 \@@_end_expandable_defaults:nnnNNn { #3 {#1} } }
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
%
% \subsection{Normalizing the argument specifications}
%
% The goal here is to expand aliases and check that the argument
% specification is valid before the main parsing run.  If it is not
% valid the entire set up is abandoned to avoid any strange internal
% errors.  A function is provided for each argument type that will grab
% any extra data items and call the loop function after performing the
% following checks and tasks.
% \begin{itemize}
%   \item Check that each argument has the correct number of data items
%     associated with it, and that where a single character is required,
%     one has actually been supplied.
%   \item Check that processors and the markers~|+|, |!| and~|=| are followed
%     by an argument for which they make sense, and are not redundant.
%   \item Check the absence of forbidden types for expandable commands,
%     namely \texttt{G}/\texttt{v} always, and \texttt{l}/\texttt{u}
%     after optional arguments (\pkg{xparse} may have inserted braces
%     due to a failed search for an optional argument).
%   \item Check that no optional argument is followed by a mandatory
%     argument with the same delimiter, as otherwise the optional
%     argument could never be omitted.
%   \item Keep track in \cs{l_@@_some_long_bool} and
%     \cs{l_@@_some_short_bool} of whether the command has some
%     long/short arguments.
%   \item Keep track in \cs{l_@@_grab_expandably_bool} of whether all
%     arguments are \texttt{m}/\texttt{l}/\texttt{u} type and short
%     arguments appear before long ones, in which case they can be
%     grabbed expandably just as safely as they could be grabbed
%     nonexpandably.  Regardless of that, arguments of expandable
%     commands will be grabbed expandably and arguments of environments
%     will not (because the list of arguments built by non-expandable
%     grabbing is used to pass them to the end-environment code).
% \end{itemize}
% Further checks happen at the end of the loop:
% \begin{itemize}
% \item that there are at most $9$ arguments;
% \item that an expandable command does not end with an optional
%   argument (this case is detected by using the fact that
%   \cs{l_@@_last_delimiters_tl} is cleared by every mandatory argument
%   and filled by every optional argument).
% \end{itemize}
%
% \begin{macro}{\@@_normalize_arg_spec:n}
% \changes{v1.3c}{2025-04-12}{Track total mandatory args}
% \begin{macro}{\@@_normalize_arg_spec_loop:n}
%   Loop through the argument specification, calling an auxiliary
%   specific to each argument type.  If any argument is unknown stop the
%   definition.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_normalize_arg_spec:n #1
  {
    \int_zero:N \l_@@_current_arg_int
    \int_zero:N \l_@@_last_mandatory_arg_int
    \tl_clear:N \l_@@_last_delimiters_tl
    \tl_clear:N \l_@@_arg_spec_tl
    \bool_set_true:N \l_@@_grab_expandably_bool
    \bool_set_false:N \l_@@_obey_spaces_bool
    \bool_set_false:N \l_@@_long_bool
    \bool_set_false:N \l_@@_suppress_strip_bool
    \bool_set_false:N \l_@@_some_obey_spaces_bool
    \bool_set_false:N \l_@@_some_long_bool
    \bool_set_false:N \l_@@_some_short_bool
    \@@_normalize_arg_spec_loop:n #1
      \q_recursion_tail \q_recursion_tail \q_recursion_tail \q_recursion_stop
    \int_compare:nNnT \l_@@_current_arg_int > 9
      {
        \msg_error:nnee { cmd } { too-many-args }
          { \@@_environment_or_command: } { \tl_to_str:n {#1} }
        \@@_bad_def:wn
      }
    \bool_if:NT \l_@@_expandable_bool
      {
        \tl_if_empty:NF \l_@@_last_delimiters_tl
          {
            \msg_error:nnee { cmd } { expandable-ending-optional }
              { \iow_char:N \\ \l_@@_function_tl } { \tl_to_str:n {#1} }
            \@@_bad_def:wn
          }
      }
    \bool_if:NT \l_@@_expandable_bool
      { \bool_set_true:N \l_@@_grab_expandably_bool }
    \bool_if:NT \l_@@_environment_bool
      { \bool_set_false:N \l_@@_grab_expandably_bool }
  }
\cs_new_protected:Npn \@@_normalize_arg_spec_loop:n #1
  {
    \quark_if_recursion_tail_stop:n {#1}
    \int_incr:N \l_@@_current_arg_int
    \cs_if_exist_use:cF { @@_normalize_type_ \tl_to_str:n {#1} :w }
      {
        \bool_lazy_any:nTF
          {
            { \str_if_eq_p:nn {#1} { G } }
            { \str_if_eq_p:nn {#1} { g } }
            { \str_if_eq_p:nn {#1} { l } }
            { \str_if_eq_p:nn {#1} { u } }
          }
          {
            \msg_error:nnee { cmd } { xparse-arg-type }
              { \@@_environment_or_command: } { \tl_to_str:n {#1} }
          }
          {
            \msg_error:nnee { cmd } { unknown-argument-type }
              { \@@_environment_or_command: } { \tl_to_str:n {#1} }
          }
        \@@_bad_def:wn
      }
  }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}
%   {
%     \@@_normalize_type_d:w,
%     \@@_normalize_type_e:w,
%     \@@_normalize_type_o:w,
%     \@@_normalize_type_O:w,
%     \@@_normalize_type_r:w,
%     \@@_normalize_type_s:w,
%   }
% \changes{v1.3h}{2025-11-04}{Update for \cs{NoValue} adjustment}
% \changes{v1.3l}{2026-04-26}{Update \cs{@@_normalize_type_r:w} to use \cs{@@_normalize_type_R_aux:w} (gh/1204)}
%   These argument types are aliases of more general ones, for example
%   with the default argument \cs{NoValue}.  For argument types that need
%   additional data, check that the data is present (not
%   \cs{q_recursion_tail}) before proceeding.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_normalize_type_d:w #1#2
  {
    \quark_if_recursion_tail_stop_do:nn {#2} { \@@_bad_arg_spec:wn }
    \@@_normalize_type_D:w {#1} {#2} { \NoValue }
  }
\cs_new_protected:Npn \@@_normalize_type_e:w #1
  {
    \quark_if_recursion_tail_stop_do:nn {#1} { \@@_bad_arg_spec:wn }
    \@@_normalize_type_E:w {#1} { }
  }
\cs_new_protected:Npn \@@_normalize_type_o:w
  { \@@_normalize_type_D:w [ ] { \NoValue } }
\cs_new_protected:Npn \@@_normalize_type_O:w
  { \@@_normalize_type_D:w [ ] }
\cs_new_protected:Npn \@@_normalize_type_r:w #1#2
  {
    \quark_if_recursion_tail_stop_do:nn {#2} { \@@_bad_arg_spec:wn }
    \@@_normalize_type_R_aux:w { r #1 #2 } {#1} {#2} { \NoValue }
  }
\cs_new_protected:Npn \@@_normalize_type_s:w
  { \@@_normalize_type_t:w * }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}
%   {
%     \@@_normalize_type_>:w,
%     \@@_normalize_type_+:w,
%     \@@_normalize_type_!:w,
%     \@@_normalize_type_=:w
%   }
% \changes{v1.1a}{2022/08/10}{Refactor to use common auxiliary}
% \changes{v1.1a}{2022/08/10}{Add support for \texttt{=} modifier}
% \begin{macro}{\@@_normalize_type_aux:NnNn}
%   Check that these prefixes have arguments, namely that the next token
%   is not \cs{q_recursion_tail}, and remember to leave it after the
%   looping macro.  Processors are forbidden in expandable commands.
%   If all is good, store the prefix in the cleaned up
%   \cs{l_@@_arg_spec_tl}, and decrement the argument number as prefixes
%   do not correspond to arguments.
%    \begin{macrocode}
\cs_new_protected:cpn { @@_normalize_type_>:w } #1#2
  {
    \quark_if_recursion_tail_stop_do:nn {#2} { \@@_bad_arg_spec:wn }
    \bool_if:NT \l_@@_expandable_bool
      {
        \msg_error:nnee { cmd } { processor-in-expandable }
          { \iow_char:N \\ \l_@@_function_tl } { \tl_to_str:n {#1} }
        \@@_bad_def:wn
      }
    \tl_put_right:Ne \l_@@_arg_spec_tl { > { \tl_trim_spaces:n {#1} } }
    \int_decr:N \l_@@_current_arg_int
    \bool_set_false:N \l_@@_grab_expandably_bool
    \@@_normalize_arg_spec_loop:n {#2}
  }
\cs_new_protected:cpn { @@_normalize_type_+:w } #1
  {
    \@@_normalize_type_aux:NnNn + {#1}
      \l_@@_long_bool
      { \bool_set_true:N \l_@@_long_bool }
  }
\cs_new_protected:cpn { @@_normalize_type_!:w } #1
  {
    \@@_normalize_type_aux:NnNn ! {#1}
      \l_@@_obey_spaces_bool
      {
        \bool_set_true:N \l_@@_obey_spaces_bool
        \bool_set_true:N \l_@@_some_obey_spaces_bool
      }
  }
\cs_new_protected:cpn { @@_normalize_type_=:w } #1#2
  {
    \@@_normalize_type_aux:NnNn = {#2}
      \l_@@_suppress_strip_bool
      {
        \bool_if:NT \l_@@_expandable_bool
          {
            \msg_error:nnee { cmd } { keyval-in-expandable }
              { \iow_char:N \\ \l_@@_function_tl } { \tl_to_str:n {#1} }
            \@@_bad_def:wn
          }
        \bool_set_true:N \l_@@_suppress_strip_bool
        \bool_set_false:N \l_@@_grab_expandably_bool
        \tl_put_right:Ne \l_@@_arg_spec_tl
          { = { \tl_trim_spaces:n {#1} } }
      }
  }
\cs_new_protected:Npn \@@_normalize_type_aux:NnNn #1#2#3#4
  {
    \quark_if_recursion_tail_stop_do:nn {#2} { \@@_bad_arg_spec:wn }
    \bool_if:NT #3
      {
        \msg_error:nnee { cmd } { two-markers }
          { \@@_environment_or_command: } { #1 }
        \@@_bad_def:wn
      }
    #4
    \int_decr:N \l_@@_current_arg_int
    \@@_normalize_arg_spec_loop:n {#2}
  }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}
%   {
%     \@@_normalize_type_D:w,
%     \@@_normalize_type_E:w,
%     \@@_normalize_type_t:w,
%   }
% \begin{macro}{\@@_normalize_E_unique_check:w}
%   Optional argument types.  Check that all required data is present
%   (and consists of single characters if applicable) and check for
%   forbidden types for expandable commands.  For \texttt{E}-type
%   require that there is at least one embellishment, that each one is a
%   single character, and that there aren't more optional arguments than
%   embellishments; also remember that each embellishment counts as one
%   argument for \cs{l_@@_current_arg_int}.  Then in each case
%   store the data in \cs{l_@@_arg_spec_tl}, and
%   for later checks store in \cs{l_@@_last_delimiters_tl} the tokens
%   whose presence determines whether there is an optional argument (for
%   braces store |{}|, seen later as an empty delimiter).
%    \begin{macrocode}
\cs_new_protected:Npn \@@_normalize_type_D:w #1#2#3
  {
    \quark_if_recursion_tail_stop_do:nn {#3} { \@@_bad_arg_spec:wn }
    \@@_single_token_check:n {#1} \@@_allowed_token_check:N #1
    \@@_single_token_check:n {#2}
    \@@_add_arg_spec:n { D #1 #2 {#3} }
    \tl_put_right:Nn \l_@@_last_delimiters_tl {#1}
    \bool_set_false:N \l_@@_grab_expandably_bool
    \@@_normalize_arg_spec_loop:n
  }
\cs_new_protected:Npn \@@_normalize_type_E:w #1#2
  {
    \quark_if_recursion_tail_stop_do:nn {#2} { \@@_bad_arg_spec:wn }
    \tl_if_blank:nT {#1} { \@@_bad_arg_spec:wn }
    \tl_map_function:nN {#1} \@@_single_token_check:n
    \tl_map_function:nN {#1} \@@_allowed_token_check:N
    \@@_normalize_E_unique_check:w #1 \q_nil \q_stop
    \int_compare:nNnT { \tl_count:n {#2} } > { \tl_count:n {#1} }
      { \@@_bad_arg_spec:wn }
    \@@_add_arg_spec:n { E {#1} {#2} }
    \tl_put_right:Nn \l_@@_last_delimiters_tl {#1}
    \bool_set_false:N \l_@@_grab_expandably_bool
    \int_add:Nn \l_@@_current_arg_int { \tl_count:n {#1} - 1 }
    \@@_normalize_arg_spec_loop:n
  }
\cs_new_protected:Npn \@@_normalize_E_unique_check:w #1#2 \q_stop
  {
    \quark_if_nil:NF #1
      {
        \tl_if_in:nnT {#2} {#1} { \@@_bad_arg_spec:wn }
        \@@_normalize_E_unique_check:w #2 \q_stop
      }
  }
\cs_new_protected:Npn \@@_normalize_type_t:w #1
  {
    \quark_if_recursion_tail_stop_do:Nn #1 { \@@_bad_arg_spec:wn }
    \@@_single_token_check:n {#1} \@@_allowed_token_check:N #1
    \tl_put_right:Ne \l_@@_arg_spec_tl
      {
        \bool_if:NT \l_@@_obey_spaces_bool { ! }
        t \exp_not:n {#1}
      }
    \tl_put_right:Nn \l_@@_last_delimiters_tl {#1}
    \bool_set_false:N \l_@@_grab_expandably_bool
    \bool_set_false:N \l_@@_obey_spaces_bool
    \bool_set_false:N \l_@@_long_bool
    \@@_normalize_arg_spec_loop:n
  }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}
%   {
%     \@@_normalize_type_m:w,
%     \@@_normalize_type_R:w,
%     \@@_normalize_type_R_aux:w,
%     \@@_normalize_type_v:w
%   }
% \changes{v1.3l}{2026-04-26}{Update \cs{@@_normalize_type_R:w} to use a
%   new auxiliary function (gh/1204)}
%   Mandatory arguments.  First check the required data is present,
%   consists of single characters where applicable, and that the argument
%   type is allowed for expandable commands if applicable.  For the
%   \texttt{m} and \texttt{R} argument types check that they do not
%   follow some optional argument with that delimiter as otherwise the
%   optional argument could not be omitted.  Then save data in
%   \cs{l_@@_arg_spec_tl}, count the mandatory argument, and empty the
%   list of last delimiters.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_normalize_type_m:w
  {
    \@@_delimiter_check:nnn { } { m } { \iow_char:N \{ }
    \@@_add_arg_spec_mandatory:n { m }
    \@@_normalize_arg_spec_loop:n
  }
\cs_new_protected:Npn \@@_normalize_type_R:w #1#2#3
  {
    \quark_if_recursion_tail_stop_do:nn {#3} { \@@_bad_arg_spec:wn }
    \@@_normalize_type_R_aux:w { R #1 #2 {#3} } {#1} {#2} {#3}
  }
\cs_new_protected:Npn \@@_normalize_type_R_aux:w #1#2#3#4
  {
    \@@_single_token_check:n {#2} \@@_allowed_token_check:N #2
    \@@_single_token_check:n {#3}
    \@@_delimiter_check:nnn {#2} { R/r } { \tl_to_str:n {#2} }
    \bool_set_false:N \l_@@_grab_expandably_bool
    \@@_add_arg_spec_mandatory:nn { R #2 #3 {#4} } {#1}
    \@@_normalize_arg_spec_loop:n
  }
\cs_new_protected:Npn \@@_normalize_type_v:w
  {
    \@@_normalize_check_gv:N v
    \@@_add_arg_spec_mandatory:n { v }
    \@@_normalize_arg_spec_loop:n
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\@@_normalize_type_b:w}
% \changes{v1.3a}{2025-01-23}{Extend to cover \texttt{c}-type grabbing}
% \begin{macro}{\@@_normalize_type_c:w}
% \changes{v1.3a}{2025-01-23}{New function}
% \begin{macro}{\@@_normalize_type_b_or_c:nn}
% \changes{v1.3a}{2025-01-23}{New function}
%   These argument types are not allowed for commands.  They are only
%   allowed at the end of the argument specification, hence we check
%   that |#1| is the end.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_normalize_type_b:w #1
  { \@@_normalize_type_b_or_c:nn {#1} { b } }
\cs_new_protected:Npn \@@_normalize_type_c:w #1
  { \@@_normalize_type_b_or_c:nn {#1} { c } }
\cs_new_protected:Npn \@@_normalize_type_b_or_c:nn #1#2
  {
    \bool_if:NF \l_@@_environment_bool
      {
        \msg_error:nnee { cmd } { invalid-command-arg }
          { \@@_environment_or_command: } {#2}
        \@@_bad_def:wn
      }
    \tl_clear:N \l_@@_last_delimiters_tl
    \@@_add_arg_spec:n {#2}
    \quark_if_recursion_tail_stop:n {#1}
    \msg_error:nneee { cmd } { arg-after-body }
      {#2}
      { \@@_environment_or_command: }
      { \tl_to_str:n {#1} }
    \@@_bad_def:wn
  }
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \end{macro}
%
% \begin{macro}{\@@_single_token_check:n}
%   Checks that the argument is a single (non-space) token (possibly
%   surrounded by spaces), and aborts the definition otherwise.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_single_token_check:n #1
  {
    \tl_trim_spaces_apply:nN {#1} \tl_if_single_token:nF
      {
        \msg_error:nnee { cmd } { not-single-token }
          { \@@_environment_or_command: } { \tl_to_str:n {#1} }
        \@@_bad_def:wn
      }
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\@@_allowed_token_check:N}
%   Some tokens are not allowed as delimiters for some argument types,
%   notably implicit begin/end-group tokens (|\bgroup|/|\egroup|).
%   The major problem with these tokens is that for |\peek_...| functions,
%   a literal~|{|$_1$ is virtually indistinguishable from a |\bgroup| or
%   other token which was |\let| to a~|{|$_1$, and the same goes
%   for~|}|$_2$.  All other tokens can be easily distinguished from their
%   implicit counterparts by grabbing them and looking at the string
%   length (see \cs{@@_token_if_cs:NTF}), but for begin/end group tokens
%   that is not possible without the risk of mistakenly grabbing the
%   entire brace group (potentially leading to a~\texttt{!~Runaway argument}
%   error) or trying to grab a |}|$_2$, leading to
%   \iffalse{\fi an~\verb|! Argument of \dots has an extra }| error.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_allowed_token_check:N #1
  {
    \token_if_eq_meaning:NNTF #1 \c_group_begin_token
        { \use:n }
        {
          \token_if_eq_meaning:NNTF #1 \c_group_end_token
            { \use:n }
            { \use_none:n }
        }
      {
        \msg_error:nneee { cmd } { forbidden-group-token }
          { \@@_environment_or_command: } { \tl_to_str:n {#1} }
          {
            \token_if_eq_meaning:NNTF #1 \c_group_begin_token
              { begin } { end }
          }
        \@@_bad_def:wn
      }
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\@@_normalize_check_gv:N, \@@_normalize_check_lu:N}
%   Called for arguments that are always forbidden, or forbidden after
%   an optional argument, for expandable commands.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_normalize_check_gv:N #1
  {
    \bool_if:NT \l_@@_expandable_bool
      {
        \msg_error:nnee { cmd } { invalid-expandable-arg }
          { \iow_char:N \\ \l_@@_function_tl } { \tl_to_str:n {#1} }
        \@@_bad_def:wn
      }
    \bool_set_false:N \l_@@_grab_expandably_bool
  }
\cs_new_protected:Npn \@@_normalize_check_lu:N #1
  {
    \bool_if:NT \l_@@_expandable_bool
      {
        \tl_if_empty:NF \l_@@_last_delimiters_tl
          {
            \msg_error:nnee { cmd } { invalid-after-optional-expandably }
              { \iow_char:N \\ \l_@@_function_tl } { \tl_to_str:n {#1} }
            \@@_bad_def:wn
          }
      }
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\@@_delimiter_check:nnn}
%   Called for \texttt{m} and \texttt{R} arguments.  Checks that the
%   leading token does not coincide with the token denoting the presence
%   of a previous optional argument.  Instead of dealing with braces for
%   the \texttt{m}-type we use an empty delimiter to denote that case.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_delimiter_check:nnn #1#2#3
  {
    \tl_map_inline:Nn \l_@@_last_delimiters_tl
      {
        \tl_if_eq:nnT {##1} {#1}
          {
            \msg_warning:nnee { cmd } { optional-mandatory }
              {#2} {#3}
          }
      }
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\@@_bad_arg_spec:wn, \@@_bad_def:wn}
%   If the argument specification is wrong, this provides an escape from
%   the entire definition process.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_bad_arg_spec:wn #1 \@@_break_point:n #2
  {
    \msg_error:nnee { cmd } { bad-arg-spec }
      { \@@_environment_or_command: } { \tl_to_str:n {#2} }
  }
\cs_new_protected:Npn \@@_bad_def:wn #1 \@@_break_point:n #2 { }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{
%   \@@_add_arg_spec:n,
%   \@@_add_arg_spec_mandatory:n,
%   \@@_add_arg_spec_mandatory:nn
% }
%  \changes{v1.2c}{2023/12/22}
%          {Clarify error message when \texttt{!} prefix is applied to
%           non-trailing opt-arg (gh/1198)}
% \changes{v1.3c}{2025-04-12}{Track total mandatory args}
% \changes{v1.3f}{2025-09-23}{Correct logic for recording position of
%   mandatory args}
% \changes{v1.3l}{2026-04-26}{Update mandatory spec function to use
%   two arguments (gh/1204)}
%   When adding an argument to the argument specification, set the
%   \texttt{some_long} or \texttt{some_short} booleans as appropriate
%   and clear the booleans keeping track of |+|, |!| and |=| markers.
%   Before that, test for a short argument following some long
%   arguments: this is forbidden for expandable commands and prevents
%   grabbing arguments expandably.
%
%   For mandatory arguments do some more work, in particular complain if
%   they were preceded by~|!|.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_add_arg_spec:n #1
  {
    \bool_lazy_and:nnT
      { ! \l_@@_long_bool }
      { \l_@@_some_long_bool }
      {
        \bool_if:NT \l_@@_expandable_bool
          {
            \msg_error:nne { cmd } { long-short-mix }
              { \iow_char:N \\ \l_@@_function_tl }
            \@@_bad_def:wn
          }
        \bool_set_false:N \l_@@_grab_expandably_bool
      }
    \bool_if:NTF \l_@@_long_bool
      { \bool_set_true:N \l_@@_some_long_bool }
      { \bool_set_true:N \l_@@_some_short_bool }
    \tl_put_right:Ne \l_@@_arg_spec_tl
      {
        \bool_lazy_and:nnT
          { \l_@@_long_bool }
          { ! \str_if_eq_p:nn {#1} { c } }
          { + }
        \bool_if:NT \l_@@_obey_spaces_bool { ! }
        \exp_not:n {#1}
      }
    \bool_set_false:N \l_@@_long_bool
    \bool_set_false:N \l_@@_obey_spaces_bool
  }
\cs_new_protected:Npn \@@_add_arg_spec_mandatory:n #1
  { \@@_add_arg_spec_mandatory:nn { #1 } { #1 } }
\cs_new_protected:Npn \@@_add_arg_spec_mandatory:nn #1 #2
  {
    \bool_if:NT \l_@@_some_obey_spaces_bool
      {
        \msg_error:nnee { cmd } { invalid-bang }
          { \@@_environment_or_command: }
          {
            \bool_if:NTF \l_@@_obey_spaces_bool
              { \tl_to_str:n {'#2'} }
              { an~optional~argument~before~mandatory~ \tl_to_str:n {'#2'} }
          }
        \@@_bad_def:wn
      }
    \tl_clear:N \l_@@_last_delimiters_tl
    \int_set_eq:NN \l_@@_last_mandatory_arg_int \l_@@_current_arg_int
    \@@_add_arg_spec:n {#1}
  }
%    \end{macrocode}
% \end{macro}
%
% \subsection{Preparing the signature: general mechanism}
%
% \begin{macro}{\@@_prepare_signature:n}
% \changes{v1.3a}{2025-01-23}{Extend to cover \texttt{c}-type grabbing}
% \begin{macro}{\@@_prepare_signature_verb_chk:n}
% \changes{v1.3n}{2026-07-28}{Test last character as a string}
% \begin{macro}{\@@_prepare_signature:N}
% \begin{macro}{\@@_prepare_signature_bypass:N}
%   Actually creating the signature uses the same loop approach as
%   normalizing the signature. There are first a number of variables which need
%   to be set to track what is going on. Many of these variables are unused
%   when defining expandable commands.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_prepare_signature:n #1
  {
    \int_set_eq:NN \l_@@_total_args_int \l_@@_current_arg_int
    \int_zero:N \l_@@_current_arg_int
    \bool_set_false:N \l_@@_long_bool
    \bool_set_false:N \l_@@_obey_spaces_bool
    \bool_set_false:N \l_@@_suppress_strip_bool
    \int_zero:N \l_@@_m_args_int
    \bool_set_false:N \l_@@_defaults_bool
    \tl_clear:N \l_@@_defaults_tl
    \tl_clear:N \l_@@_process_all_tl
    \tl_clear:N \l_@@_process_one_tl
    \bool_set_false:N \l_@@_process_some_bool
    \tl_clear:N \l_@@_signature_tl
    \bool_set_false:N \l_@@_final_verb_bool
    \@@_prepare_signature_verb_chk:n {#1}
    \@@_prepare_signature:N #1 \q_recursion_tail \q_recursion_stop
    \bool_if:NF \l_@@_expandable_bool { \@@_flush_m_args: }
  }
%    \end{macrocode}
%   A quick check on the final arg.~type, done using a string approach  (that retains braces) to
%   avoid an issue with \verb|{c}| as the last balanced text.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_prepare_signature_verb_chk:n #1
  {
    \tl_if_in:eeT
      { \tl_trim_spaces:e { \tl_to_str:n {#1} } \exp_not:N \q_stop }
      { \tl_to_str:n { c } \exp_not:N \q_stop }
      { \bool_set_true:N \l_@@_final_verb_bool }
  }
%    \end{macrocode}
%  The main looping function does not take an argument, but carries out the
%  reset on the processor boolean. This is split off from the rest of the
%  process so that when actually setting up processors the flag-reset can
%  be bypassed.
%
%  For each known argument type there is an appropriate function to actually
%  do the addition to the signature. These are separate for expandable and
%  standard functions, as the approaches are different.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_prepare_signature:N
  {
    \bool_set_false:N \l_@@_prefixed_bool
    \@@_prepare_signature_bypass:N
  }
\cs_new_protected:Npn \@@_prepare_signature_bypass:N #1
  {
    \quark_if_recursion_tail_stop:N #1
    \use:c
      {
         @@_add
         \bool_if:NT \l_@@_grab_expandably_bool { _expandable }
         _type_  \token_to_str:N #1 :w
      }
  }
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
%
% \subsection{Setting up a standard signature}
%
% Each argument-adding function appends to the signature a grabber (and
% for some types, the delimiters or default value), except the one for
% \texttt{m} arguments.  These are collected and added to the signature
% all at once by \cs{@@_flush_m_args:}, called for every other argument
% type.  All of the functions then call the loop function
% \cs{@@_prepare_signature:N}.  Default values of arguments are
% collected by \cs{@@_add_default:n} rather than being stored with the
% argument; this function and \cs{@@_add_default:} are also responsible
% for keeping track of \cs{l_@@_current_arg_int}.
%
% \begin{macro}{\@@_add_type_+:w}
%   Making the next argument long means setting the flag. The \texttt{m}
%   arguments are recorded here as
%   this has to be done for every case where there is then a long argument.
%    \begin{macrocode}
\cs_new_protected:cpn { @@_add_type_+:w }
  {
    \@@_flush_m_args:
    \bool_set_true:N \l_@@_long_bool
    \bool_set_true:N \l_@@_prefixed_bool
    \@@_prepare_signature_bypass:N
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\@@_add_type_!:w}
%   Much the same for controlling trailing optional arguments.
%    \begin{macrocode}
\cs_new_protected:cpn { @@_add_type_!:w }
  {
    \@@_flush_m_args:
    \bool_set_true:N \l_@@_obey_spaces_bool
    \bool_set_true:N \l_@@_prefixed_bool
    \@@_prepare_signature_bypass:N
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\@@_add_type_>:w}
%   When a processor is found, the processor code is stored.  It will be
%   used by \cs{@@_args_process:} once arguments are all found. Here too
%   the loop calls \cs{@@_prepare_signature_bypass:N} rather than
%   \cs{@@_prepare_signature:N} so that the flag is not reset.
%    \begin{macrocode}
\cs_new_protected:cpn { @@_add_type_>:w } #1
  {
    \@@_flush_m_args:
    \bool_set_true:N \l_@@_prefixed_bool
    \bool_set_true:N \l_@@_process_some_bool
    \tl_put_left:Nn \l_@@_process_one_tl { {#1} }
    \@@_prepare_signature_bypass:N
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\@@_add_type_=:w}
% \changes{v1.1a}{2022/08/10}{Add support for \texttt{=} modifier}
%   A mix of the ideas from above: set a flag and add a processor.
%    \begin{macrocode}
\cs_new_protected:cpn { @@_add_type_=:w } #1
  {
    \@@_flush_m_args:
    \bool_set_true:N \l_@@_prefixed_bool
    \bool_set_true:N \l_@@_suppress_strip_bool
    \bool_set_true:N \l_@@_process_some_bool
    \tl_put_left:Nn \l_@@_process_one_tl
      { { \@@_arg_to_keyvalue:nn {#1} } }
    \@@_prepare_signature_bypass:N
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\@@_add_type_b:w}
% \changes{v1.3a}{2025-01-23}{Extend to cover \texttt{c}-type grabbing}
% \begin{macro}{\@@_add_type_c:w}
% \changes{v1.3a}{2025-01-23}{New function}
% \begin{macro}{\@@_add_type_b_or_c:N}
% \changes{v1.3a}{2025-01-23}{New function}
%    \begin{macrocode}
\cs_new_protected:Npn \@@_add_type_b:w
  { \@@_add_type_b_or_c:N b }
\cs_new_protected:Npn \@@_add_type_c:w
  { \@@_add_type_b_or_c:N c }
\cs_new_protected:Npn \@@_add_type_b_or_c:N #1
  {
    \@@_flush_m_args:
    \@@_add_default:
    \@@_add_grabber:N #1
    \@@_prepare_signature:N
  }
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \end{macro}
%
% \begin{macro}{\@@_add_type_D:w}
%    \begin{macrocode}
\cs_new_protected:Npn \@@_add_type_D:w #1#2#3
  {
    \@@_flush_m_args:
    \@@_add_default:n {#3}
    \@@_add_grabber:N D
    \tl_put_right:Nn \l_@@_signature_tl { #1 #2 }
    \@@_prepare_signature:N
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\@@_add_type_E:w}
%   The \texttt{E}-type argument needs a special handling of default
%   values.  Since each embellishment is a separate argument, it
%   also needs to replicate the argument processors for each embellishment
%   argument so that the numbers of arguments and processors remain in sync.
% \changes{v1.0g}{2021/08/07}
%         {Replicate argument processors for all embellishments (gh/639)}
%    \begin{macrocode}
\cs_new_protected:Npn \@@_add_type_E:w #1#2
  {
    \@@_flush_m_args:
    \@@_add_default_E:nn {#1} {#2}
    \use:e
      {
        \@@_replicate_processor:nn { \tl_count:n {#1} }
          { \exp_not:o \l_@@_process_one_tl }
      }
    \@@_add_grabber:N E
    \tl_put_right:Nn \l_@@_signature_tl { {#1} }
    \@@_prepare_signature:N
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\@@_replicate_processor:nn}
%   In the command's argument processor signature (the final argument of
%   \cs{@@_start:nNNnnn}) there is one braced item for each formal
%   argument (up to nine), and in each of these items there is one
%   braced item for each processor (as many as there were processors
%   declared for a given argument).  Something like this:
%   \begin{quote}
%     \ttfamily \obeylines
%     \{ \% argument processors
%     \ \ \{ \% argument 1
%     \ \ \ \ \{ processor 1 \} \{ processor 2 \} \ldots\ \{ processor n \}
%     \ \ \} \% end argument 1
%     \ \ \{ \ldots\ \} \% argument 2
%     \ \ \ \ \ \vdots
%     \ \ \{ \ldots\ \} \% argument n
%     \} \% end argument processors
%   \end{quote}
%
%   The function \cs{@@_add_grabber:N} adds one single grabber for an
%   argument, and adds the braced item for that one argument.  However,
%   in an |E|-type argument each embellishment requires its own formal
%   argument, so we need to break out of one layer of braces in
%   \cs{l_@@_process_one_tl}, add copies of the processor as necessary,
%   and then return the removed brace.  The function below does just
%   that:  it defines \cs{l_@@_process_one_tl} starting with a
%   \iffalse{\fi |}|$_2$
%   and ending with a |{|$_1$, \iffalse}\fi
%   so that it adds as many processors as
%   needed when |x|-expanded.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_replicate_processor:nn #1 #2
  {
    \int_compare:nNnF {#1} > { 1 } { \use_none:nnn }
    \tl_set:Ne \l_@@_process_one_tl
      {
        \exp_not:n { \exp_not:n {#2} \if_false: { \fi: } }
        \prg_replicate:nn { #1 - 2 }
          { \exp_not:n { \exp_not:n { {#2} } } }
        \exp_not:n { { \if_false: } \fi: \exp_not:n {#2} }
      }
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\@@_add_type_m:w}
%   The \texttt{m} type is special as short arguments which are not
%   post-processed are simply counted at this stage. Thus there is a check
%   to see if either of these cases apply. If so, a one-argument grabber
%   is added to the signature. On the other hand, if a standard short
%   argument is required it is simply counted at this stage, to be
%   added later using \cs{@@_flush_m_args:}.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_add_type_m:w
  {
    \@@_add_default:
    \bool_if:NTF \l_@@_prefixed_bool
      { \@@_add_grabber:N m }
      { \int_incr:N \l_@@_m_args_int }
    \@@_prepare_signature:N
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\@@_add_type_R:w}
%   The \texttt{R}-type argument is very similar to the \texttt{D}-type.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_add_type_R:w #1#2#3
  {
    \@@_flush_m_args:
    \@@_add_default:n {#3}
    \@@_add_grabber:N R
    \tl_put_right:Nn \l_@@_signature_tl { #1 #2 }
    \@@_prepare_signature:N
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\@@_add_type_t:w}
%   Setting up a \texttt{t} argument means collecting one token for the test,
%   and adding it along with the grabber to the signature.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_add_type_t:w #1
  {
    \@@_flush_m_args:
    \@@_add_default:
    \@@_add_grabber:N t
    \tl_put_right:Nn \l_@@_signature_tl {#1}
    \@@_prepare_signature:N
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\@@_add_type_v:w}
% \changes{v1.3h}{2025-11-04}{Update for \cs{NoValue} adjustment}
%   At this stage, the \texttt{v} argument is identical to \texttt{l}
%   except that since the grabber may fail to read a verbatim argument
%   we need a default value.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_add_type_v:w
  {
    \@@_flush_m_args:
    \@@_add_default:n { \NoValue }
    \@@_add_grabber:N v
    \@@_prepare_signature:N
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\@@_flush_m_args:}
%   As \texttt{m} arguments are simply counted, there is a need to add
%   them to the token register in a block. As this function can only
%   be called if something other than \texttt{m} turns up, the flag can
%   be switched here.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_flush_m_args:
  {
    \int_compare:nNnT \l_@@_m_args_int > 0
      {
        \tl_put_right:Ne \l_@@_signature_tl
          { \exp_not:c { @@_grab_m_ \int_use:N \l_@@_m_args_int :w } }
        \tl_put_right:Ne \l_@@_process_all_tl
          { \prg_replicate:nn { \l_@@_m_args_int } { { } } }
      }
    \int_zero:N \l_@@_m_args_int
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\@@_add_grabber:N}
%   To keep the various checks needed in one place, adding the grabber
%   to the signature is done here.  The only questions are whether the
%   grabber should be long or not, and whether to obey spaces.  The
%   \cs{l_@@_obey_spaces_bool} boolean can only be \texttt{true} for
%   trailing optional arguments.  In that case spaces will not be
%   ignored when looking for that optional argument.
% \changes{v1.0g}{2021/08/07}
%         {Replicate argument processors for all embellishments (gh/639)}
% \changes{v1.3a}{2025-01-23}{Extend to support \texttt{c}-type grabbing}
% \changes{v1.3c}{2025-04-12}{Refine handling of optional arguments for
%   \texttt{c}-type grabbing}
%    \begin{macrocode}
\cs_new_protected:Npn \@@_add_grabber:N #1
  {
    \tl_put_right:Ne \l_@@_signature_tl
      {
        \exp_not:c
          {
            @@_grab_ #1
            \bool_if:NT \l_@@_long_bool { _long }
            \bool_if:NT \l_@@_obey_spaces_bool { _obey_spaces }
            \bool_lazy_and:nnT
              { \l_@@_suppress_strip_bool }
              { \str_if_eq_p:nn {#1} { D } }
              { _no_strip }
            \bool_lazy_all:nT
              {
                { \l_@@_final_verb_bool }
                { \str_if_eq_p:nn {#1} { D } }
                {
                  \int_compare_p:nNn \l_@@_current_arg_int 
                    > \l_@@_last_mandatory_arg_int
                }
              }
              { _verb_safe }
            :w
          }
      }
    \bool_set_false:N \l_@@_long_bool
    \bool_set_false:N \l_@@_obey_spaces_bool
    \bool_set_false:N \l_@@_suppress_strip_bool
    \bool_set_false:N \l_@@_verb_safe_bool
    \tl_put_right:Ne \l_@@_process_all_tl
      {
        {
          \if_charcode:w E #1 \use_i:nn \fi:
          \exp_not:o \l_@@_process_one_tl
        }
      }
    \tl_clear:N \l_@@_process_one_tl
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\@@_add_default:n, \@@_add_default:, \@@_add_default_E:nn}
% \changes{v1.3h}{2025-11-04}{Update for \cs{NoValue} adjustment}
%   Store the default value of an argument, or rather code that gives
%   that default value (it may involve other arguments).  This is
%    (in a brace group) \cs{prg_do_nothing:}
%   followed by a default value or \cs{NoValue}.  For \texttt{E}-type
%   arguments, pad the defaults |#2| with some \cs{NoValue}
%   until there are as many as embellishments~|#1|.  These functions are
%   also used when defining expandable commands. The \cs{prg_do_nothing:} here
%   are removed in \cs{@@_defaults_def:nnn}: for a user-supplied default the
%   token is injected to avoid brace stripping, and for \cs{NoValue} means
%   we have a common code path later.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_add_default:n #1
  {
    \tl_if_novalue:nTF {#1}
      { \@@_add_default: }
      {
        \int_incr:N \l_@@_current_arg_int
        \bool_set_true:N \l_@@_defaults_bool
        \tl_put_right:Nn \l_@@_defaults_tl { { \prg_do_nothing: #1 } }
      }
  }
\cs_new_protected:Npn \@@_add_default:
  {
    \int_incr:N \l_@@_current_arg_int
    \tl_put_right:Nn \l_@@_defaults_tl { { \prg_do_nothing: \NoValue } }
  }
\cs_new_protected:Npn \@@_add_default_E:nn #1#2
  {
    \tl_map_function:nN {#2} \@@_add_default:n
    \prg_replicate:nn
      { \tl_count:n {#1} - \tl_count:n {#2} }
      { \@@_add_default: }
  }
%    \end{macrocode}
% \end{macro}
%
% \subsection{Setting up expandable types}
%
% The approach here is not dissimilar to that for standard types, but fewer types
% are supported. There is
% also a need to define the per-function auxiliaries: this is done here, while
% the general grabbers are dealt with later.
%
% \begin{macro}{\@@_add_expandable_type_+:w}
%   We have already checked that short arguments are before long
%   arguments, so \cs{l_@@_long_bool} only changes from \texttt{false}
%   to \texttt{true} once (and there is no need to reset it after each
%   argument).  Continue the loop.
%    \begin{macrocode}
\cs_new_protected:cpn { @@_add_expandable_type_+:w }
  {
    \bool_set_true:N \l_@@_long_bool
    \@@_prepare_signature:N
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\@@_add_expandable_type_D:w}
% \begin{macro}{\@@_add_expandable_type_D_aux:NNNn}
% \begin{macro}{\@@_add_expandable_type_D_aux:NNN}
% \begin{macro}{\@@_add_expandable_type_D_aux:NN}
%   The set up for \texttt{D}-type arguments involves constructing a
%   rather complex auxiliary which is used
%   repeatedly when grabbing. There is an auxiliary here so that the
%   \texttt{R}-type can share code readily: |#1| is |D| or~|R|.
%   The |_aux:NN| auxiliary is needed if the two delimiting tokens are
%   identical: in contrast to the non-expandable route, the grabber here
%   has to act differently for this case.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_add_expandable_type_D:w
  { \@@_add_expandable_type_D_aux:NNNn D }
\cs_new_protected:Npn \@@_add_expandable_type_D_aux:NNNn #1#2#3#4
  {
    \@@_add_default:n {#4}
    \tl_if_eq:nnTF {#2} {#3}
      { \@@_add_expandable_type_D_aux:NN #1 #2 }
      { \@@_add_expandable_type_D_aux:NNN #1 #2 #3 }
    \@@_prepare_signature:N
  }
\cs_new_protected:Npn \@@_add_expandable_type_D_aux:NNN #1#2#3
  {
    \bool_if:NTF \l_@@_long_bool
      { \cs_set:cpe }
      { \cs_set_nopar:cpe }
      { \l_@@_expandable_aux_name_tl } ##1 ##2 #2 ##3 \q_@@ ##4 #3
      { ##1 {##2} {##3} {##4} }
    \@@_add_expandable_grabber:nn {#1}
      {
        \exp_not:c  { \l_@@_expandable_aux_name_tl }
        \exp_not:n { #2 #3 }
      }
  }
\cs_new_protected:Npn \@@_add_expandable_type_D_aux:NN #1#2
  {
    \bool_if:NTF \l_@@_long_bool
      { \cs_set:cpe }
      { \cs_set_nopar:cpe }
      { \l_@@_expandable_aux_name_tl } ##1 #2 ##2 #2
      { ##1 {##2} }
    \@@_add_expandable_grabber:nn { #1_alt }
      {
        \exp_not:c  { \l_@@_expandable_aux_name_tl }
        \exp_not:n {#2}
      }
  }
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
%
% \begin{macro}{\@@_add_expandable_type_E:w}
% \changes{v1.3h}{2025-11-04}{Update for \cs{NoValue} adjustment}
% \begin{macro}{\@@_add_expandable_type_E_aux:n}
%   For each embellishment, use \cs{@@_get_grabber:NN} to obtain an
%   auxiliary delimited by that token and store a pair constituted of
%   the auxiliary and the token in \cs{l_@@_tmpb_tl}, before appending
%   the whole set of these pairs to the signature, and an equal number
%   of \cs{NoValue} markers (regardless of the default values of
%   arguments).  Set the current argument appropriately.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_add_expandable_type_E:w #1#2
  {
    \@@_add_default_E:nn {#1} {#2}
    \tl_clear:N \l_@@_tmpb_tl
    \tl_map_function:nN {#1} \@@_add_expandable_type_E_aux:n
    \@@_add_expandable_grabber:nn
      { E \bool_if:NT \l_@@_long_bool { _long } }
      {
        { \exp_not:o \l_@@_tmpb_tl }
        {
          \prg_replicate:nn { \tl_count:n {#1} }
            { { \NoValue } }
        }
      }
    \@@_prepare_signature:N
  }
\cs_new_protected:Npn \@@_add_expandable_type_E_aux:n #1
  {
    \@@_get_grabber:NN #1 \l_@@_tmpa_tl
    \tl_put_right:Ne \l_@@_tmpb_tl
      { \exp_not:o \l_@@_tmpa_tl \exp_not:N #1 }
  }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}{\@@_add_expandable_type_m:w}
%   Unlike the standard case, when working expandably each argument is always
%   grabbed separately.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_add_expandable_type_m:w
  {
    \@@_add_default:
    \@@_add_expandable_grabber:nn
      { m \bool_if:NT \l_@@_long_bool { _long } } { }
    \@@_prepare_signature:N
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\@@_add_expandable_type_R:w}
%   The \texttt{R}-type is very similar to the \texttt{D}-type
%   argument, and so the same internals are used.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_add_expandable_type_R:w
  { \@@_add_expandable_type_D_aux:NNNn R }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\@@_add_expandable_type_t:w}
%   An auxiliary delimited by |#1| is built now.  It will be used to
%   test for the presence of that token.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_add_expandable_type_t:w #1
  {
    \@@_add_default:
    \@@_get_grabber:NN #1 \l_@@_tmpa_tl
    \@@_add_expandable_grabber:nn { t }
      {
        \exp_not:o \l_@@_tmpa_tl
        \exp_not:N #1
      }
    \@@_prepare_signature:N
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\@@_add_expandable_grabber:nn}
%   This is called for all arguments to place the right grabber in the
%   signature.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_add_expandable_grabber:nn #1#2
  {
    \tl_put_right:Ne \l_@@_signature_tl
      { \exp_not:c { @@_expandable_grab_ #1 :w } #2 }
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\@@_get_grabber:NN}
% \begin{macro}{\@@_get_grabber_auxi:NN}
% \begin{macro}{\@@_get_grabber_auxii:NN}
%   Given a token |#1|, defines an expandable function delimited by that
%   token and stores it in the token list~|#2|.  The function is named
%   after the token, unless that function name is already taken by some
%   other grabber (this can happen in the rare case where delimiters
%   with different category codes are used in the same document): in
%   that case use a global counter to get a unique name.  Since the
%   grabbers are not named after \pkg{xparse} commands they should not
%   be used to get material from the input stream.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_get_grabber:NN #1#2
  {
    \cs_set:Npn \@@_tmp:w ##1 #1 {##1}
    \exp_args:Nc \@@_get_grabber_auxi:NN
      { @@_grabber_ \token_to_str:N #1 :w } #2
  }
\cs_new_protected:Npn \@@_get_grabber_auxi:NN #1#2
  {
    \cs_if_eq:NNTF \@@_tmp:w #1
      { \tl_set:Nn #2 {#1} }
      {
        \cs_if_exist:NTF #1
          {
            \int_gincr:N \g_@@_grabber_int
            \exp_args:Nc \@@_get_grabber_auxi:NN
              {
                @@_grabber_
                - \int_use:N \g_@@_grabber_int :w
              }
              #2
          }
          { \@@_get_grabber_auxii:NN #1 #2 }
      }
  }
\cs_new_protected:Npn \@@_get_grabber_auxii:NN #1#2
  {
    \cs_set_eq:NN #1 \@@_tmp:w
    \tl_set:Nn #2 {#1}
  }
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \end{macro}
%
% \subsubsection{Copying a command and its internal structure}
%
%    \begin{macrocode}
%<latexrelease>\IncludeInRelease{2021/11/15}{\@@_copy:NN}%
%<latexrelease>  {Support~\NewCommandCopy~in~ltcmd}
%    \end{macrocode}
%
% \changes{v1.0h}{2021/08/30}{Added support for \cs{NewCommandCopy}}
% Since the 2020-10-01 \LaTeXe{} release, support for copying, and
% showing the definition of, robust commands has been available, but the
% specifics of each command are implemented separately.  Here we'll add
% support for copying and showing \pkg{ltcmd} definitions.
%
% To fully support copying, we need two commands:  a conditional to test
% if a command is in fact a \pkg{ltcmd} command, and another command to
% actually copy the command.  The conditional is defined later as
% \cs{__kernel_cmd_if_xparse:NTF}, so now to the copying:
%
% \begin{macro}{\@@_copy:NN}
% \begin{macro}{\@@_set_eq_if_exist:NN,\@@_set_eq_if_exist:cc}
%   This macro just branches to the proper copying command by using
%   \cs{@@_cmd_type_cases:NnnnnnF}.  The copying command takes the names
%   of the commands to be copied to and from, and the actual commands
%   as its four arguments.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_copy:NN #1 #2
  {
    \use:e
      {
        \int_set:Nn \tex_escapechar:D { 92 }
        \exp_not:N \@@_cmd_type_cases:NnnnnnF \exp_not:N #2
          { \@@_copy_command:nnNN }
          { \@@_copy_expandable:nnNN }
          { \@@_copy_optimized:nnNN }
          { \@@_copy_environment:nnNN }
          { \@@_copy_environment_end:nnNN }
          { \@@_cant_copy:nwn { non-ltcmd } }
            { \cs_to_str:N #1 } { \cs_to_str:N #2 }
            \exp_not:N #1 \exp_not:N #2
        \exp_not:N \@@_break_point:n { \cs_to_str:N #2 }
        \int_set:Nn \tex_escapechar:D { \int_use:N \tex_escapechar:D }
      }
  }
\cs_new_protected:Npn \@@_set_eq_if_exist:NN #1 #2
  { \cs_if_exist:NTF #2 { \cs_set_eq:NN } { \use_none:nn } #1 #2 }
\cs_generate_variant:Nn \@@_set_eq_if_exist:NN { cc }
%    \end{macrocode}
%
% \begin{macro}{\@@_cant_copy:nwn}
%   An utility macro similar to \cs{@@_bad_def:wn} to abort a command
%   copy.  Contrary to \cs{@@_bad_def:wn} though, when this happens the
%   issue is most likely internal, because the command was already
%   (supposedly) correctly defined so it should be copyable.  Hopefully
%   this macro will never be used ever, but if it does, apologize and
%   give the reason for the failure so the user can report.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_cant_copy:nwn #1 #2 \@@_break_point:n #3
  { \msg_error:nnnn { cmd } { copy-bug } {#1} {#3} }
\msg_new:nnn { cmd } { copy-bug }
  {
    Error~while~copying~command~\iow_char:N\\#2:\\
    \str_case:nn {#1}
      {
        { non-ltcmd } { Command~is~not~a~valid~ltcmd~command. }
        { unknown-type } { Found~an~unknown~argument~type. }
        { invalid-end }
          { Target~command~is~not~named~\iow_char:N \\end<name>. }
      }
  }
%    \end{macrocode}
% \end{macro}
%
%   And, of course, add \cs{__kernel_cmd_if_xparse:NTF} and
%   \cs{@@_copy:NN} to \cs{@declarecommandcopylisthook}:
%    \begin{macrocode}
\tl_gput_right:Nn \@declarecommandcopylisthook
  { { \__kernel_cmd_if_xparse:NTF \@@_copy:NN } }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}{\@@_copy_command:nnNN,\@@_copy_command:NnNNnnnn}
%   A normal (non-expandable) command has a pretty straightforward
%   structure.  Its definition is stored in
%   \cs{\meta{cmd}\textvisiblespace code}, its defaults (if any) are
%   stored in \cs{\meta{cmd}\textvisiblespace defaults}, and its
%   top-level definition contains its signature, which can just be
%   copied over.  \cs{@@_copy_command:nnNN} copies the command code and
%   defaults, and then defines the top-level command using the auxiliary
%   \cs{@@_copy_command:NnNNnnnn}.  This macro takes the signature of
%   the command being copied from its top-level definition, and replaces
%   the named bits with the new name.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_copy_command:nnNN #1 #2 #3 #4
  {
    \cs_set_eq:cc { #1 ~ code } { #2 ~ code }
    \@@_set_eq_if_exist:cc { #1 ~ defaults } { #2 ~ defaults }
    \cs_set_protected_nopar:Npe #3
      { \exp_after:wN \@@_copy_command:NnNNnnnn #4 {#1} }
  }
\cs_new:Npn \@@_copy_command:NnNNnnnn #1 #2 #3 #4 #5 #6 #7 #8
  {
    #1 \exp_not:n { {#2} }
    \exp_not:c { #8 ~ } \exp_not:c { #8 ~ code }
    \exp_not:n { {#5} {#6} {#7} }
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\@@_copy_expandable:nnNN,\@@_copy_expandable:NnNNNNnnn}
%   An expandable command is slightly more complicated.  Besides the
%   \cs{\meta{cmd}\textvisiblespace code}, and
%   \cs{\meta{cmd}\textvisiblespace defaults}, it also has an auxiliary
%   \cs{\meta{cmd}\textvisiblespace} for grabbing delimited arguments,
%   and possibly another auxiliary
%   \cs{\meta{cmd}\textvisiblespace\textvisiblespace}, if the command
%   has both long and short arguments.  Then, its signature also has
%   several specific bits that are unique to that command; this is in contrast
%   to non-expandable commands, which use a common set of parsing functions.
%
%   We start by copying the basics, then call
%   \cs{@@_copy_expandable_signature:NnNNNNnnn} to parse the signature
%   of the command and build up the modified copy in a temporary token list,
%   then we call \cs{@@_copy_expandable:NnNNNNnnn} that will copy the
%   top-level definition of the command, with the proper internal
%   renames.
%
%    \begin{macrocode}
%<latexrelease>\EndIncludeInRelease
%<latexrelease>\IncludeInRelease{2020/10/01}{\@@_copy:NN}%
%<latexrelease>  {Support~\NewCommandCopy~in~ltcmd}
%<latexrelease>\EndIncludeInRelease
%    \end{macrocode}
%
% \changes{v1.1c}{2023/03/12}
%         {Distinguish (non-expandable) document commands starting with
%          \cs{__cmd_start_expandable:nNNNNn}}
%   There's one variant: a command begins with \cs{@@_start_expandable:nNNNNn}
%   may still be un-expandable/protected if it's defined by 
%   \cs{NewDocumentCommand} and friends, with empty or only m-type arguments.
%
%    \begin{macrocode}
%<latexrelease>\IncludeInRelease{2023/06/01}{\@@_copy_expandable:nnNN}%
%<latexrelease>  {Distinguish~non-expandable~document~commands}
\cs_new_protected:Npn \@@_copy_expandable:nnNN #1 #2 #3 #4
  {
    \cs_set_eq:cc { #1 ~ code } { #2 ~ code }
    \@@_set_eq_if_exist:cc { #1 ~ } { #2 ~ }
    \@@_set_eq_if_exist:cc { #1 ~ \c_space_tl } { #2 ~ \c_space_tl }
    \@@_set_eq_if_exist:cc { #1 ~ defaults } { #2 ~ defaults }
    \exp_after:wN \@@_copy_expandable_signature:NnNNNNnnn #4 {#1} {#2}
    \token_if_protected_macro:NTF #4
      { \cs_set_protected_nopar:Npe }{ \cs_set_nopar:Npe }
      #3
      { \exp_after:wN \@@_copy_expandable:NnNNNNnnn #4 {#1} {#2} }
  }
%<latexrelease>\EndIncludeInRelease
%<latexrelease>\IncludeInRelease{2021/11/15}{\@@_copy_expandable:nnNN}%
%<latexrelease>  {Support~\NewCommandCopy~in~ltcmd}
%<latexrelease>\cs_new_protected:Npn \@@_copy_expandable:nnNN #1 #2 #3 #4
%<latexrelease>  {
%<latexrelease>    \cs_set_eq:cc { #1 ~ code } { #2 ~ code }
%<latexrelease>    \@@_set_eq_if_exist:cc { #1 ~ } { #2 ~ }
%<latexrelease>    \@@_set_eq_if_exist:cc { #1 ~ \c_space_tl } { #2 ~ \c_space_tl }
%<latexrelease>    \@@_set_eq_if_exist:cc { #1 ~ defaults } { #2 ~ defaults }
%<latexrelease>    \exp_after:wN \@@_copy_expandable_signature:NnNNNNnnn #4 {#1} {#2}
%<latexrelease>    \cs_set_nopar:Npx #3
%<latexrelease>      { \exp_after:wN \@@_copy_expandable:NnNNNNnnn #4 {#1} {#2} }
%<latexrelease>  }
%<latexrelease>\EndIncludeInRelease
%<latexrelease>\IncludeInRelease{2020/10/01}{\@@_copy_expandable:nnNN}%
%<latexrelease>  {Support~\NewCommandCopy~in~ltcmd}
%<latexrelease>\EndIncludeInRelease
%    \end{macrocode}
%
% \begin{macro}{\@@_copy_optimized:nnNN}
%   Copy the code, simply define the wrapper.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_copy_optimized:nnNN #1#2#3#4
  {
    \cs_set_eq:cc { #1 ~ code } { #2 ~ code }
    \token_if_protected_macro:NTF #4
      { \cs_set_protected_nopar:Npe }
      { \cs_set_nopar:Npe }
        #3
        {
          \exp_not:N \@@_start_optimized:
          \exp_not:c { #1 ~ code }
        }
  }
%    \end{macrocode}
% \end{macro}
%
%    \begin{macrocode}
%<latexrelease>\IncludeInRelease{2021/11/15}{\@@_copy:NN (part 2)}%
%<latexrelease>  {Support~\NewCommandCopy~in~ltcmd}
%    \end{macrocode}
%
%    \begin{macrocode}
\cs_new:Npn \@@_copy_expandable:NnNNNNnnn #1 #2 #3 #4 #5 #6 #7 #8 #9
  {
    \exp_not:N #1 \exp_not:n { {#2} }
    \exp_not:c { #8 ~ }
    \exp_not:c
      {
        #8 ~
        \str_if_eq:eeT
            { \exp_not:c { #9 ~ \c_space_tl } } { \exp_not:N #4 }
          { \c_space_tl }
      }
    \exp_not:c { #8 ~ code }
    \str_if_eq:eeTF { \exp_not:N #6 } { ? }
      { ? }
      { \exp_not:c { #8 ~ defaults } }
    { \exp_not:V \l_@@_tmpa_tl }
  }
%    \end{macrocode}
%
% \begin{macro}{
%     \@@_copy_expandable_signature:NnNNNNnnn,
%     \@@_copy_expandable:nnN,
%     \@@_copy_parse_grabber:w,
%   }
%   A signature for an expandable command contains as many
%   \cs{expandable_grab_\meta{type}:w} as there are arguments, and what
%   follows this macro depends on the \meta{type}.  We'll start a loop
%   through the signature, and at each argument grabber, we'll step the
%   argument count, and look for the \meta{type} with
%   \cs{@@_copy_parse_grabber:w} so that we know which
%   \cs{@@_copy_grabber_\meta{type}:w} to call next.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_copy_expandable_signature:NnNNNNnnn
    #1 #2 #3 #4 #5 #6 #7 #8 #9
  {
    \int_zero:N \l_@@_current_arg_int
    \tl_clear:N \l_@@_tmpa_tl
    \@@_copy_expandable:nnN {#8} {#9} #7
      \q_recursion_tail \q_recursion_stop
  }
\cs_new_protected:Npn \@@_copy_expandable:nnN #1 #2 #3
  {
    \quark_if_recursion_tail_stop:n {#3}
    \int_incr:N \l_@@_current_arg_int
    \exp_after:wN \@@_copy_parse_grabber:w \token_to_str:N #3 {#1} {#2}
  }
\use:e
  {
    \cs_new_protected:Npn \exp_not:N \@@_copy_parse_grabber:w #1
        \tl_to_str:n { expandable_grab_ } #2 \tl_to_str:n { :w }
      {
        \tl_put_right:Ne \exp_not:N \l_@@_tmpa_tl
          { \exp_not:N \exp_not:c { @@_expandable_grab_#2:w } }
        \exp_not:N \cs_if_exist_use:cF { @@_copy_grabber_#2:w }
          { \@@_cant_copy:nwn { unknown-type } }
      }
  }
%    \end{macrocode}
%
% \begin{macro}{
%     \@@_copy_grabber_D:w,\@@_copy_grabber_D_alt:w,
%     \@@_copy_grabber_R:w,\@@_copy_grabber_R_alt:w,
%     \@@_copy_grabber_E:w,\@@_copy_grabber_E_long:w,
%     \@@_copy_grabber_t:w,
%     \@@_copy_grabber_m:w,\@@_copy_grabber_m_long:w,
%   }
%   The most complicated is the |D|elimited argument:  each argument has
%   a dedicated grabbing function named after the command that has to be
%   copied over (of the form
%   \cs{\meta{cmd}\textvisiblespace(arg\textvisiblespace\meta{num})}).
%    \begin{macrocode}
\cs_new_protected:Npn \@@_copy_grabber_D:w #1 #2 #3 #4 #5
  {
    \tl_put_right:Ne \l_@@_tmpa_tl
      {
        \exp_not:c { #1 ~ (arg ~ \int_use:N \l_@@_current_arg_int ) }
        \exp_not:n { #4 #5 }
      }
    \cs_set_eq:cc
      { #1 ~ (arg ~ \int_use:N \l_@@_current_arg_int ) }
      { #2 ~ (arg ~ \int_use:N \l_@@_current_arg_int ) }
    \@@_copy_expandable:nnN {#1} {#2}
  }
%    \end{macrocode}
%
%   |D_alt| is just a special case of |D| that uses a single delimiter
%   (used when both delimiters of the argument are identical):
%    \begin{macrocode}
\cs_new_protected:Npn \@@_copy_grabber_D_alt:w #1 #2 #3 #4
  { \@@_copy_grabber_D:w {#1} {#2} {#3} {#4} { } }
%    \end{macrocode}
%
%   As far as copying is concerned, |R| is identical to |D|:
%    \begin{macrocode}
\cs_new_eq:NN \@@_copy_grabber_R:w \@@_copy_grabber_D:w
\cs_new_eq:NN \@@_copy_grabber_R_alt:w \@@_copy_grabber_D_alt:w
%    \end{macrocode}
%
%   |E| is straightforward: we just copy the embellishments over, and
%   increase the current argument number \cs{l_@@_current_arg_int} by
%   the number of embellishments (minus one because there is a
%   \cs{int_incr:N} down the line).
%    \begin{macrocode}
\cs_new_protected:Npn \@@_copy_grabber_E:w #1 #2 #3 #4
  {
    \tl_put_right:Nn \l_@@_tmpa_tl { {#3} {#4} }
    \int_add:Nn \l_@@_current_arg_int { \tl_count:n {#4} - 1 }
    \@@_copy_expandable:nnN {#1} {#2}
  }
\cs_new_eq:NN \@@_copy_grabber_E_long:w \@@_copy_grabber_E:w
%    \end{macrocode}
%
%   |t| just needs copying the token to be tested for:
%    \begin{macrocode}
\cs_new_protected:Npn \@@_copy_grabber_t:w #1 #2 #3 #4
  {
    \tl_put_right:Nn \l_@@_tmpa_tl { #3 #4 }
    \@@_copy_expandable:nnN {#1} {#2}
  }
%    \end{macrocode}
%
%   And last but not least, |m| is the simplest;  the grabber is just
%   \cs{@@_expandable_grab_m:w}, which is already added to the new
%   command so here we just resume the loop:
%    \begin{macrocode}
\cs_new_protected:Npn \@@_copy_grabber_m:w { \@@_copy_expandable:nnN }
\cs_new_eq:NN \@@_copy_grabber_m_long:w \@@_copy_grabber_m:w
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \end{macro}
%
% \begin{macro}{\@@_copy_environment:nnNN}
% \begin{macro}{\@@_copy_environment:Nnnnnnn}
%   Copying an environment's \cs{begin} part is pretty much like copying
%   a command, except it has a longer name, and at the end we have to
%   copy \cs[no-index]{environment~\meta{name}} into
%   \cs[no-index]{\meta{name}}.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_copy_environment:nnNN #1 #2 #3 #4
  {
    \cs_set_eq:cc { environment~ #1 ~ code } { environment~ #2 ~ code }
    \@@_set_eq_if_exist:cc
      { environment~ #1 ~ defaults } { environment~ #2 ~ defaults }
    \cs_set_protected_nopar:cpe { environment~ #1 }
      { \exp_after:wN \@@_copy_environment:Nnnnnnn #4 {#1} }
    \cs_set_eq:cc {#1} { environment~ #1 }
  }
\cs_new:Npn \@@_copy_environment:Nnnnnnn #1 #2 #3 #4 #5 #6 #7
  { #1 \exp_not:n { {#2} } {#7} \exp_not:n { {#4} {#5} {#6} } }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}{\@@_copy_environment_end:nnNN}
% \begin{macro}{\@@_copy_environment_end_aux:nnNN}
%   Copying an environment's \cs{end} part is a bit trickier.  We first
%   have to make sure that both parts are named
%   \cs[no-index]{end\meta{name}} (that's actually not a hard
%   requirement, but an environment \cs{end} command makes no sense
%   without the |end| in its name), and strip the leading |end| from the
%   strings.  After that, copying is straightforward.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_copy_environment_end:nnNN #1 #2
  {
    \@@_check_end:Nn \l_@@_tmpa_tl {#1}
    \@@_check_end:Nn \l_@@_tmpb_tl {#2}
    \exp_args:Noo \@@_copy_environment_end_aux:nnNN
      { \l_@@_tmpa_tl } { \l_@@_tmpb_tl }
  }
\cs_new_protected:Npn \@@_copy_environment_end_aux:nnNN #1 #2 #3 #4
  {
    \cs_set_nopar:cpe { environment~ #1 ~end }
      { \exp_not:c { environment~ #1 ~end~aux } }
    \cs_set_eq:cc
      { environment~ #1 ~end~aux~ } { environment~ #2 ~end~aux~ }
    \cs_set_eq:cc { end #1 } { environment~ #1 ~end }
  }
%    \end{macrocode}
%
% \begin{macro}{\@@_check_end:Nn,\@@_check_end:n,\@@_check_end:w}
%   To check whether an \cs{end} command is valid, we look for the
%   string |end| at the beginning of the command name, and if not found,
%   raise an error:
%    \begin{macrocode}
\cs_new_protected:Npn \@@_check_end:Nn #1 #2
  {
    \tl_set:Ne #1 { \@@_check_end:n {#2} }
    \token_if_eq_meaning:NNT #1 \q_nil
      { \@@_cant_copy:nwn { invalid-end } }
  }
\cs_set_protected:Npn \@@_tmp:w #1
  {
    \cs_new:Npn \@@_check_end:n ##1
      {
        \exp_after:wN \@@_check_end:w \tl_to_str:n {##1}
          #1 \q_mark #1 \q_stop
      }
    \cs_new:Npn \@@_check_end:w ##1 #1 ##2 #1 ##3 \q_stop
      { \if_meaning:w ##2 \q_mark \exp_not:N \q_nil \else: ##2 \fi: }
  }
\exp_args:No \@@_tmp:w { \tl_to_str:n { end } }
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \end{macro}
%
% Not much to do regarding \pkg{latexrelease}: we could remove the
% entries from \cs{@declarecommandcopylisthook}, but it doesn't seem
% worth it.
%    \begin{macrocode}
%<latexrelease>\EndIncludeInRelease
%<latexrelease>\IncludeInRelease{2020/10/01}{\@@_copy:NN (part 2)}%
%<latexrelease>  {Support~\NewCommandCopy~in~ltcmd}
%<latexrelease>\EndIncludeInRelease
%    \end{macrocode}
%
% \subsubsection{Showing the definition of a command}
%
%    \begin{macrocode}
%<latexrelease>\IncludeInRelease{2021/11/15}{\@@_show:N}%
%<latexrelease>  {Support~\ShowCommand~in~ltcmd}
%    \end{macrocode}
%
% \changes{v1.0h}{2021/08/30}{Added support for \cs{ShowCommand}}
% To show the definition of a command we need more or less the same
% building blocks as for copying, except that instead of making a copy,
% we'll just print stuff to the terminal.
%
% \begin{macro}{\@@_show:N}
%   This macro just branches to the proper showing command by using
%   \cs{@@_cmd_type_cases:NnnnnnF}.  The showing command takes the command
%   to be shown as argument.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_show:N #1
  {
    \use:e
      {
        \int_set:Nn \tex_escapechar:D { 92 }
        \exp_not:N \@@_cmd_type_cases:NnnnnnF \exp_not:N #1
          { \@@_show_command:N }
          { \@@_show_expandable:N }
          { \@@_show_optimized:N }
          { \@@_show_environment:N }
          { \@@_show_environment_end:N }
          { \@@_cant_copy:nwn { non-ltcmd } }
            \exp_not:N #1
        \exp_not:N \@@_break_point:n { \cs_to_str:N #1 }
        \int_set:Nn \tex_escapechar:D { \int_use:N \tex_escapechar:D }
      }
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{
%     \@@_show_command:N,
%     \@@_show_command:NnNNwN,
%     \@@_show_expandable:N,
%     \@@_show_expandable:NnNNNNnN,
%     \@@_show_optimized:N,
%     \@@_show_command_aux:NnNNn,
%     \@@_show_environment:N,
%     \@@_show:e,
%   }
%   These commands just expand the command once to reveal its innards,
%   then pass the type of command, the control sequence, the signature,
%   and the code macro to \cs{@@_show_command_aux:NnNNn}.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_show_command:N #1
  { \exp_after:wN \@@_show_command:NnNNwN #1 \q_@@ #1 }
\cs_new_protected:Npn \@@_show_command:NnNNwN #1 #2 #3 #4 #5 \q_@@ #6
  {
    \@@_show_command_aux:NnNNn \tl_show:e
      { document~command } #6 #4 {#2}
  }
\cs_new_protected:Npn \@@_show_expandable:N #1
  { \exp_after:wN \@@_show_expandable:NnNNNNnN #1 #1 }
%    \end{macrocode}
%
%    \begin{macrocode}
%<latexrelease>\EndIncludeInRelease
%<latexrelease>\IncludeInRelease{2020/10/01}{\@@_show:N}%
%<latexrelease>  {Support~\ShowCommand~in~ltcmd}
%<latexrelease>\EndIncludeInRelease
%    \end{macrocode}
%
% \changes{v1.1c}{2023/03/12}
%         {Distinguish (non-expandable) document commands starting with
%          \cs{__cmd_start_expandable:nNNNNn}}
%   There's one variant: a command begins with \cs{@@_start_expandable:nNNNNn}
%   may still be un-expandable/protected if it's defined by 
%   \cs{NewDocumentCommand} and friends, with empty or only m-type arguments.
%    \begin{macrocode}
%<latexrelease>\IncludeInRelease{2023/06/01}{\@@_show_expandable:NnNNNNnN}%
%<latexrelease>  {Distinguish~non-expandable~document~commands}
\cs_new_protected:Npn \@@_show_expandable:NnNNNNnN #1 #2 #3 #4 #5 #6 #7 #8
  {
    \exp_args:NNe \@@_show_command_aux:NnNNn \tl_show:e
      { \token_if_protected_macro:NF #8 { expandable~ } document~command }
      #8 #5 {#2}
  }
%<latexrelease>\EndIncludeInRelease
%<latexrelease>\IncludeInRelease{2021/11/15}{\@@_show_expandable:NnNNNNnN}%
%<latexrelease>  {Support~\ShowCommand~in~ltcmd}
%<latexrelease>\cs_new_protected:Npn \@@_show_expandable:NnNNNNnN #1 #2 #3 #4 #5 #6 #7 #8
%<latexrelease>  {
%<latexrelease>    \@@_show_command_aux:NnNNn \tl_show:x
%<latexrelease>      { expandable~document~command } #8 #5 {#2}
%<latexrelease>  }
%<latexrelease>\EndIncludeInRelease
%<latexrelease>\IncludeInRelease{2020/10/01}{\@@_show_expandable:NnNNNNnN}%
%<latexrelease>  {Support~\ShowCommand~in~ltcmd}
%<latexrelease>\EndIncludeInRelease
%    \end{macrocode}
%
%    \begin{macrocode}
%<latexrelease>\IncludeInRelease{2021/11/15}{\@@_show:N (part 2)}%
%<latexrelease>  {Support~\ShowCommand~in~ltcmd}
%    \end{macrocode}
%
%   Now just print everything in the required format.  The auxiliary
%   \cs{@@_split_signature:n} stores a ready-to-print token list in
%   \cs{l_@@_tmpa_tl}, so we ust use that here:
% \changes{v1.0j}{2021/13/31}
%         {Make \cs{ShowCommand} stop for interaction}
% \changes{v1.1b}{2022/11/30}
%         {Don't stop for the \cs{begin} part of an environment}
%    \begin{macrocode}
\cs_new_protected:Npn \@@_show_command_aux:NnNNn #1 #2 #3 #4 #5
  {
    \@@_split_signature:n {#5}
    #1
      {
        \token_to_str:N #3 = #2: \iow_newline:
        \tl_use:N \l_@@_tmpa_tl
        -> \cs_replacement_spec:N #4
      }
  }
%    \end{macrocode}
%
% Optimized functions need things done a bit differently as we need to
% reconstruct the argument spec.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_show_optimized:N #1
  {
    \exp_args:Nc \@@_show_optimized:NN
      { \cs_to_str:N #1 \c_space_tl code }
      #1
  }
\cs_new_protected:Npn \@@_show_optimized:NN #1#2
  {
    \cs_set:Npe \@@_show_optimized_aux:N ##1
      {
        \c_space_tl \c_space_tl \c_hash_str ##1 :
        \bool_lazy_or:nnT
          { \token_if_long_macro_p:N #1 }
          { \token_if_protected_long_macro_p:N #1 }
            { + } m
         \iow_newline:
      }
    \tl_show:e
      {
        \token_to_str:N #2 =
          \bool_lazy_or:nnF
            { \token_if_protected_macro_p:N #1 }
            { \token_if_protected_long_macro_p:N #1 }
              { expandable ~ } document~command:
        \iow_newline:
        \int_step_function:nN
          {
            \int_div_truncate:nn
              { \tl_count:e { \cs_parameter_spec:N #1 } }
              { 2 }
          }
          \@@_show_optimized_aux:N
        ->
        \cs_replacement_spec:N #1
      }
  }
%    \end{macrocode}
%
%   We can reuse most of the above to show an environment, except that
%   we need to ensure that the proper \cs[no-index]{environment~\ldots}
%   are passed to \cs{@@_show_command_aux:NnNNn}.  Additionally, when
%   |\ShowCommand\foo| is used (if |foo| is an environment), we show
%   |\endfoo| as well, and when |\ShowCommand\endfoo| is used, change
%   that to |\ShowCommand\foo| and do the same.
% \changes{v1.0j}{2021/13/31}
%         {Make \cs{ShowCommand} stop for interaction}
% \changes{v1.1b}{2022/11/30}
%         {Don't stop for the \cs{begin} part of an environment}
% \changes{v1.3e}{2025-06-27}{Support \cs{showstream} (gh/1062)}
%    \begin{macrocode}
\cs_new_protected:Npn \@@_show_environment:N #1
  {
    \exp_after:wN \@@_show_environment:Nnnw #1 \q_@@
    \tl_show:e
      {
        \token_to_str:N \end { \cs_to_str:N #1 } : \iow_newline:
        -> \exp_args:Nc \cs_replacement_spec:N
          { environment~ \cs_to_str:N #1 ~end~aux~ }
      }
  }
\cs_new_protected:Npn \@@_show_environment:Nnnw #1 #2 #3 #4 \q_@@
  {
    \use:e
      {
        \@@_show_command_aux:NnNNn \@@_show:e { document~environment }
          { \exp_not:N \begin {#3} }
          \exp_not:c { environment~ #3 ~ code }
          {#2}
      }
  }
\cs_if_exist:NTF \iow_show:e
  {
    \cs_new_protected:Npn \@@_show:e #1
      { \iow_show:e { > ~ #1 . \iow_newline: } }
  }
  {
    \cs_new_protected:Npn \@@_show:e #1
      { \iow_term:e { > ~ #1 . \iow_newline: } }
  }
\cs_new_protected:Npn \@@_show_environment_end:N #1
  {
    \exp_args:NNe \@@_check_end:Nn \l_@@_tmpa_tl { \cs_to_str:N #1 }
    \exp_args:Nc \@@_show_environment:N { \l_@@_tmpa_tl }
  }
%    \end{macrocode}
%
%   And, of course, add \cs{__kernel_cmd_if_xparse:NTF} and
%   \cs{@@_show:N} to \cs{@showcommandlisthook} and to
%   \cs{@showenvironmentlisthook} (\cs{@@_show:N} takes care of the
%   environment case as well, so both entries are identical):
%  \changes{v1.1b}{2022-11-29}{Add \cs{@showenvironmentlisthook}}
%    \begin{macrocode}
\tl_gput_right:Nn \@showcommandlisthook
  { { \__kernel_cmd_if_xparse:NTF \@@_show:N } }
\tl_gput_right:Nn \@showenvironmentlisthook
  { { \__kernel_cmd_if_xparse:NTF \@@_show:N } }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\@@_split_signature:n}
%   Now we'll try a least-effort adventure into splitting the symbolic
%   user-provided signature for a command into individual parameters for
%   pretty-printing.  A counter is used to keep track of the current
%   argument number, and two token lists are used: \cs{l_@@_tmpa_tl}
%   holds the final token list to be printed, and \cs{l_@@_tmpb_tl}
%   holds just the current item, so that we can make changes to an
%   individual item without having to dissect the whole thing (this is
%   used for |e|- and |E|-types).
%    \begin{macrocode}
\cs_new_protected:Npn \@@_split_signature:n #1
  {
    \int_set:Nn \l_@@_current_arg_int { 1 }
    \tl_clear:N \l_@@_tmpa_tl
    \tl_clear:N \l_@@_tmpb_tl
    \@@_split_signature_loop:Nw #1 \q_recursion_tail \q_recursion_stop
  }
%    \end{macrocode}
%
% \begin{macro}{\@@_split_signature_loop:Nw}
%   This is the main chunk of the loop:  it starts an item with
%   \cs{@@_split_start_item:} 
%   (this adds indentation and the argument number to
%   \cs{l_@@_tmpb_tl}), then checks if a special token list
%   \cs[no-index]{c_@@_show_type_\meta{type}_tl} exists.  If it doesn't,
%   the current argument is a \enquote{simple} type which needs no extra
%   processing.  Otherwise, call a specific function depending on the
%   value of said token list.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_split_signature_loop:Nw #1
  {
    \quark_if_recursion_tail_stop:N #1
    \tl_if_empty:NT \l_@@_tmpb_tl { \@@_split_start_item: }
    \tl_if_exist:cTF { c_@@_show_type_#1_tl }
      {
        \use:c
          {
            @@_show_
            \if_case:w \tl_use:c { c_@@_show_type_#1_tl } \exp_stop_f:
            delim \or: delims \or: delims_opt \or: opt \or:
            e \or: E \or: prefix \or: processor \fi: :Nw
          } #1
      }
      { \@@_split_end_item:n {#1} \@@_split_signature_loop:Nw }
  }
%    \end{macrocode}
%
% \begin{macro}{
%     \c_@@_show_type_t_tl,
%     \c_@@_show_type_r_tl,\c_@@_show_type_d_tl,
%     \c_@@_show_type_R_tl,\c_@@_show_type_D_tl,
%     \c_@@_show_type_O_tl,
%     \c_@@_show_type_e_tl,
%     \c_@@_show_type_E_tl,
%     \c_@@_show_type_+_tl,\c_@@_show_type_!_tl,
%     \c_@@_show_type_>_tl,
%   }
%   The token lists \cs[no-index]{c_@@_show_type_\meta{type}_tl} exist
%   for nontrivial (for printing) \meta{types} that require special
%   parsing (like delimiters or optional arguments).  Values from~0 to~7
%   are assigned to each type:
%   \begin{enumerate}
%     \item a single delimiter token;
%     \item two delimiter tokens;
%     \item two delimiter tokens plus a default value;
%     \item a default value;
%     \item a list of embellishments (exclusive for |e|-type);
%     \item embellishments plus defaults (exclusive for |E|-type);
%     \item simple prefixes;
%     \item prefixes with arguments (argument processors);
%   \end{enumerate}
%    \begin{macrocode}
\cs_set_protected:Npn \@@_tmp:w #1 #2
  {
    \quark_if_nil:nF {#1}
      { \tl_const:cn { c_@@_show_type_#1_tl } {#2} \@@_tmp:w }
  }
\@@_tmp:w t0 r1 d1 R2 D2 O3 e4 E5 +6 !6 >7 =7 \q_nil \q_nil
%    \end{macrocode}
%
% \begin{macro}{
%     \@@_show_delim:Nw,\@@_show_delims:Nw,
%     \@@_show_delims_opt:Nw,\@@_show_opt:Nw,
%     \@@_show_e:Nw,\@@_show_E:Nw,
%     \@@_show_prefix:Nw,\@@_show_processor:Nw,
%   }
% \changes{v1.3h}{2025-11-04}{Update for \cs{NoValue} adjustment}
%   Now, based on each type we know how to act.  In most cases it is
%   just a matter of feeding in the grabbed arguments and resuming the
%   loop.  The embellishments require a bit more attention:  the
%   |e|-type loops through the list of embellishments and adds each to
%   the token list as a separate argument.  The |E|-type does more or
%   less the same, but uses \cs{@@_tl_mapthread_function:nnN} to map
%   over two lists simultaneously, adding each token and default to the
%   token list for printing.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_show_delim:Nw #1 #2
  { \@@_split_end_item:n { #1 #2 } \@@_split_signature_loop:Nw }
\cs_new_protected:Npn \@@_show_delims:Nw #1 #2 #3
  { \@@_split_end_item:n { #1 #2 #3 } \@@_split_signature_loop:Nw }
\cs_new_protected:Npn \@@_show_delims_opt:Nw #1 #2 #3 #4
  { \@@_split_end_item:n { #1 #2 #3 {#4} } \@@_split_signature_loop:Nw }
\cs_new_protected:Npn \@@_show_opt:Nw #1 #2
  { \@@_split_end_item:n { #1 {#2} } \@@_split_signature_loop:Nw }
\cs_new_protected:Npn \@@_show_e:Nw #1 #2
  {
    \tl_map_inline:nn {#2}
      {
        \@@_split_start_item:
        \@@_split_end_item:n { #1 ##1 }
      }
    \@@_split_signature_loop:Nw
  }
\cs_set_protected:Npn \@@_tmp:w #1
  {
    \cs_new_protected:Npn \@@_show_E:Nw ##1 ##2 ##3
      {
        \cs_set_protected:Npn \@@_tmp:w ####1 ####2
          {
            \@@_split_start_item:
            \@@_split_end_item:n { ##1 ####1 {####2} }
          }
        \@@_tl_mapthread_function:nnN {##2}
          { ##3 {#1} {#1} {#1} {#1} {#1} {#1} {#1} {#1} {#1} } \@@_tmp:w
        \@@_split_signature_loop:Nw
      }
  }
\@@_tmp:w \NoValue
%    \end{macrocode}
%
%   Minor wrinkle with the prefixes: they use \cs{@@_split_add_item:n}
%   instead of \cs{@@_split_end_item:n} (|add| \emph{vs.} |end|) because
%   they are followed by an argument, so they can't end the item.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_show_prefix:Nw #1
  { \@@_split_add_item:n {#1} \@@_split_signature_loop:Nw }
\cs_new_protected:Npn \@@_show_processor:Nw #1 #2
  { \@@_split_add_item:n { #1 {#2} } \@@_split_signature_loop:Nw }
%    \end{macrocode}
%
% \begin{macro}{
%     \@@_split_start_item:,
%     \@@_split_add_item:n,
%     \@@_split_end_item:n,
%   }
%   And now the auxiliaries that store the strings to be printed.
%   \cs{@@_split_start_item:} starts an item from scratch,
%   \cs{@@_split_add_item:n} adds tokens to an item without adding a
%   newline, and \cs{@@_split_end_item:n} adds tokens, terminates the
%   item with a newline, and steps the argument count.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_split_start_item:
  {
    \tl_set:Ne \l_@@_tmpb_tl
      { ~ \c_space_tl \c_hash_str \int_use:N \l_@@_current_arg_int : }
  }
\cs_new_protected:Npn \@@_split_add_item:n #1
  { \tl_put_right:Ne \l_@@_tmpb_tl { \tl_to_str:n {#1} } }
\cs_new_protected:Npn \@@_split_end_item:n #1
  {
    \tl_put_right:Ne \l_@@_tmpa_tl
      { \l_@@_tmpb_tl \tl_to_str:n {#1} \iow_newline: }
    \tl_clear:N \l_@@_tmpb_tl
    \int_incr:N \l_@@_current_arg_int
  }
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
%
% Not much to do regarding \pkg{latexrelease}: we could remove the
% entries from \cs{@showcommandlisthook}, but it doesn't seem
% worth it.
%    \begin{macrocode}
%<latexrelease>\EndIncludeInRelease
%
%<latexrelease>\IncludeInRelease{2020/10/01}{\@@_show:N (part 2)}%
%<latexrelease>  {Support~\ShowCommand~in~ltcmd}
%<latexrelease>\EndIncludeInRelease
%    \end{macrocode}
%
% \subsection{Grabbing arguments}
%
% All of the grabbers follow the same basic pattern. The initial
% function stores in \cs{l_@@_signature_tl} the code to grab further
% arguments, defines (the function in) \cs{l_@@_fn_tl} that will grab
% the argument, and calls it.
%
% Defining \cs{l_@@_fn_tl} means determining whether to use
% \cs{cs_set:Npn} or \cs{cs_set_nopar:Npn}, and for optional arguments
% whether to skip spaces. Once the argument is found, \cs{l_@@_fn_tl}
% calls \cs{@@_add_arg:n}, responsible for calling processors and
% grabbing further arguments.
%
% \begin{macro}
%   {
%     \@@_grab_b:w,
%     \@@_grab_b_long:w,
%     \@@_grab_b_obey_spaces:w,
%     \@@_grab_b_long_obey_spaces:w,
%     \@@_grab_b_aux:NNw,
%     \@@_grab_b_end:Nw
%   }
% \changes{v1.1a}{2022/08/10}{Track changes in \texttt{D}-type implementation}
%   This uses the well-tested code of \texttt{D}-type arguments,
%   skipping the peeking step because the \texttt{b}-type argument is
%   always present, and adding a cleanup stage at the end by hijacking
%   the signature.  The clean-up consists of properly
%   dealing with \cs{l_@@_args_tl} and also putting back the \cs{end}
%   that served as an end-delimiter: this \cs{end} receives the
%   environment name as its argument and is run normally.  The
%   \texttt{D}-type code stores the argument found (body of the
%   environment) as a brace group in \cs{l_@@_args_tl} and depending on
%   the presence of a prefix~|!| we trim spaces or not before adding
%   this braced argument into the saved \cs{l_@@_args_tl}.
%   The strange \verb*|\begin | control sequence is there for display
%     purposes only: it has to look like |\begin| in the terminal but
%       not to delimited arguments.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_grab_b:w
  { \@@_grab_b_aux:NNw \cs_set_protected_nopar:Npn \tl_trim_spaces:n }
\cs_new_protected:Npn \@@_grab_b_long:w
  { \@@_grab_b_aux:NNw \cs_set_protected:Npn \tl_trim_spaces:n }
\cs_new_protected:Npn \@@_grab_b_obey_spaces:w
  { \@@_grab_b_aux:NNw \cs_set_protected_nopar:Npn \exp_not:n }
\cs_new_protected:Npn \@@_grab_b_long_obey_spaces:w
  { \@@_grab_b_aux:NNw \cs_set_protected:Npn \exp_not:n }
\cs_new_protected:Npn \@@_grab_b_aux:NNw #1#2#3 \@@_run_code:
  {
    \@@_grab_D_aux:NNnNN \begin \end {#3} #1 \use_ii:nn
    \tl_put_left:Nn \l_@@_signature_tl { \@@_grab_b_end:Nw #2 }
    \tl_set_eq:NN \l_@@_saved_args_tl \l_@@_args_tl
    \tl_clear:N \l_@@_args_tl
    \exp_args:Nc \l_@@_fn_tl { begin ~ }
  }
\cs_new_protected:Npn \@@_grab_b_end:Nw #1#2 \@@_run_code:
  {
    \tl_set:Ne \l_@@_args_tl
      {
        \exp_not:V \l_@@_saved_args_tl
        { \exp_after:wN #1 \l_@@_args_tl }
      }
    #2
    \@@_run_code:
    \end
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\@@_grab_c:w}
% \changes{v1.3a}{2025-01-23}{New \texttt{c}-type grabbing function}
% \begin{macro}{\@@_grab_c_obey_spaces:w}
% \begin{macro}{\@@_grab_c_start:n}
% \begin{macro}{\@@_grab_c_first:w}
% \begin{macro}{\@@_grab_c_loop:w}
%   Collecting an environment body verbatim shares some ideas with the
%   \texttt{v}-type grabber, and others with the standard \texttt{filecontents}
%   environment. The start is to set the end-of-line to a predictable value
%   and to deactivate the specials.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_grab_c:w #1 \@@_run_code:
  {
    \bool_set_false:N \l_@@_obey_spaces_bool
    \@@_grab_c_start:n {#1}
  }
\cs_new_protected:Npn \@@_grab_c_obey_spaces:w #1 \@@_run_code:
  {
    \bool_set_true:N \l_@@_obey_spaces_bool
    \@@_grab_c_start:n {#1}
  }
\cs_new_protected:Npn \@@_grab_c_start:n #1
  {
    \tl_set:Nn \l_@@_signature_tl {#1}
    \group_begin:
      \tl_clear:N \l_@@_v_arg_tl
      \tex_escapechar:D = 92 \scan_stop:
      \tex_endlinechar:D = `\^^M \scan_stop:
      \cs_set_eq:NN \do \char_set_catcode_other:N
      \dospecials
      \char_set_catcode_other:n { `\^^M }
      \@@_grab_c_first:w
  }
%    \end{macrocode}
% \changes{v1.3c}{2025-04-12}{Correct behavior on dropping tokens}
%   Notice here and below that we cannot use |\token_to_str:N \end| as that
%   would have the wrong category codes for the letters.
%    \begin{macrocode}
\group_begin:
  \char_set_catcode_other:N \^^M %
  \cs_new_protected:Npn \@@_grab_c_first:w #1 ^^M %
    { %
      \tl_if_blank:nTF {#1} %
        { %
          \@@_grab_c_loop:w #1 ^^M %
        } %
        { %
          \msg_warning:nnee { cmd } { chars-dropped-first-line }  %
            { \exp_not:n {#1} }  %
            { \exp_not:V \@currenvir } %
           \@@_grab_c_loop:w ^^M %
        } %
    } %
  \cs_new_protected:Npe \@@_grab_c_loop:w #1 ^^M %
    { %
      \exp_not:N \@@_grab_c_auxi:w #1 %
        \c_backslash_str end %
        \scan_stop: %
    } %
\group_end:
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
% \begin{macro}{\@@_grab_c_auxi:w}
% \begin{macro}{\@@_grab_c_auxii:w}
% \begin{macro}{\@@_grab_c_auxiii:N}
% \begin{macro}{\@@_grab_c_auxiv:}
% \begin{macro}{\@@_grab_c_auxv:}
% \begin{macro}{\@@_grab_c_auxvi:N}
% \begin{macro}{\@@_grab_c_auxvii:}
% \begin{macro}{\@@_grab_c_auxviii:}
%   We need to see if the current line contains |\end| followed by the name
%   of the current environment. To do that and allow for spaces, we have to
%   work stepwise. First, establish if there is an |\end| at all: remember that
%   here we are dealing with \enquote{other} tokens. Whether these is an
%   |\end| or not, the tokens \emph{before} it form part of the line.
%    \begin{macrocode}
\use:e
  {
    \cs_new_protected:Npn \exp_not:N \@@_grab_c_auxi:w
      #1 \c_backslash_str end #2 \scan_stop:
  }
  {
    \tl_put_right:Nn \l_@@_v_arg_tl {#1}
    \tl_if_empty:nTF {#2}
      {
        \tl_put_right:Nn \l_@@_v_arg_tl { \obeyedline }
        \@@_grab_c_loop:w
      }
      { \@@_grab_c_auxii:w #2 \scan_stop: }
  }
%    \end{macrocode}
%   There is an |\end|, so we now remove the trailing marker we needed
%   to do the test. This is stripped off, then we need to examine the
%   rest of the line one token at a time: see |verbatim.dtx| for the
%   inspiration. Notice that we use |^^M| here as the end marker: this allows
%   looping to look for multiple |\end| entries in the line.
%    \begin{macrocode}
\group_begin:
  \char_set_catcode_other:N \^^M %
  \use:e %
    { %
      \cs_new_protected:Npe \exp_not:N \@@_grab_c_auxii:w %
        #1 \c_backslash_str end \scan_stop: %
    } %
    { %
      \tl_set:Nn \exp_not:N \l_@@_tmpa_tl
        { \c_backslash_str end } %
      \exp_not:N \@@_grab_c_auxiii:N #1 ^^M %
    } %
%    \end{macrocode}
% \changes{v1.3d}{2025-04-21}{Correct a logic flaw}
%   Within the line, we need to collect up the tokens: if we do not find
%   the end-of-environment argument, we will need those to reinsert. There
%   are three special cases here: |^^M| (end of line: tidy up and back to
%   the main loop), \verb*| | (possibly skip over) and |{| (start the inner
%   loop). Anything else means we move back to examine the rest of the line
%   for any more |\end| entries.
%    \begin{macrocode}
  \cs_new_protected:Npn \@@_grab_c_auxiii:N #1 %
    { %
      \token_case_charcode:NnF #1 %
        { %
          ^^M %
            { \@@_grab_c_auxiv: } %
          \c_space_token %
            { %
             \tl_put_right:Nn \l_@@_tmpa_tl {#1} %
             \@@_grab_c_auxiii:N %
            } %
          \c_group_begin_token %
            { %
              \tl_set:Nn \l_@@_tmpb_tl {#1} %
              \@@_grab_c_auxvi:N %
            } %
        } %
        { %
          \tl_put_right:Nn \l_@@_tmpa_tl {#1} %
          \@@_grab_c_auxv: %
        } %
    } %
\group_end:
\cs_new_protected:Npn \@@_grab_c_auxiv:
  {
    \tl_put_right:Ne \l_@@_v_arg_tl
      {
        \exp_not:V \l_@@_tmpa_tl
        \exp_not:N \obeyedline
      }
    \@@_grab_c_loop:w
  }
\cs_new_protected:Npn \@@_grab_c_auxv:
  {
    \tl_put_right:NV \l_@@_v_arg_tl \l_@@_tmpa_tl
    \@@_grab_c_loop:w
  }
%    \end{macrocode}
%   In the inner loop, we again have only a few special cases. First, we could
%   again have |^^M|, in which case we tidy up using a common auxiliary. Second,
%   we check for the escape char: this cannot happen inside the end of an
%   environment and means we loop, re-inserting the token. Finally, we have
%   |}|, where we need to move on to check what has been collected. Otherwise,
%   collect up and loop. Notice here that the inner loop needs to
%   collect tokens separately: this leaves any spaces after |\end| in
%   \cs{l_@@_tmpa_tl}, so we can test \cs{l_@@_tmpb_tl} directly.
%    \begin{macrocode}
\group_begin:
  \char_set_catcode_other:N \^^M %
  \cs_new_protected:Npe \@@_grab_c_auxvi:N #1 %
    { %
      \exp_not:N \token_case_charcode:NnF #1 %
        { %
          ^^M %
            {
              \exp_not:N \@@_grab_c_auxvii: %
              \exp_not:N \@@_grab_c_auxiv: %
            }%
          \c_backslash_str %
            { %
              \exp_not:N \@@_grab_c_auxvii: %
              \exp_not:N \@@_grab_c_auxv: #1
            } %
         \c_group_end_token %
           { %
             \tl_put_right:Nn \exp_not:N \l_@@_tmpb_tl {#1} %
             \exp_not:N \@@_grab_c_auxviii: %
           } %
        } %
        { %
          \tl_put_right:Nn \exp_not:N \l_@@_tmpb_tl {#1} %
          \exp_not:N \@@_grab_c_auxvi:N %
        } %
    } %
\group_end: %
\cs_new_protected:Npn \@@_grab_c_auxvii:
  { \tl_put_right:NV \l_@@_tmpa_tl \l_@@_tmpb_tl }
\cs_new_protected:Npn \@@_grab_c_auxviii:
  {
    \str_if_eq:eeTF { \exp_not:V \l_@@_tmpb_tl } { { \@currenvir } }
      { \@@_grab_c_end:w }
      {
        \@@_grab_c_auxvii:
        \@@_grab_c_auxv:
      }
  }
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
% \begin{macro}{\@@_grab_c_end:w}
% \begin{macro}{\@@_grab_c_end:n}
% \begin{macro}{\@@_grab_c_end_auxi:w}
% \begin{macro}{\@@_grab_c_end_auxii:w}
% \begin{macro}{\@@_grab_c_end_auxiii:w}
%   To end the collection, we clean up the last line: once again we need to
%   find |^^M|. Once that is done, we can warn if there is anything left
%   behind.
%    \begin{macrocode}
\group_begin:
  \char_set_catcode_other:N \^^M %
  \cs_new_protected:Npn \@@_grab_c_end:w #1 ^^M %
    { %
        \tl_if_blank:nF {#1} %
          { %
            \msg_warning:nnee { cmd } { chars-dropped-last-line }  %
              { \exp_not:n {#1} }  %
              { \exp_not:V \@currenvir } %
          } %
      \exp_args:NNNo \group_end: %
      \tl_set:Nn \l_@@_v_arg_tl { \l_@@_v_arg_tl } %
      \@@_add_arg:e %
        { %
          \bool_if:NTF \l_@@_obey_spaces_bool %
            { \exp_not:V } %
            { \exp_args:NV \@@_grab_c_end:n } %
           \l_@@_v_arg_tl %
        } %
      \exp_args:NV \end \@currenvir %
    } %
\group_end: %
%    \end{macrocode}
%   Look for line markers at each end and tidy up if required.
%    \begin{macrocode}
\cs_new:Npn \@@_grab_c_end:n #1
  {
    \@@_grab_c_end_auxi:w \q_nil #1 \q_nil
      \obeyedline \obeyedline \q_nil \q_stop
  }
\cs_new:Npn \@@_grab_c_end_auxi:w #1 \q_nil \obeyedline
  { \@@_grab_c_end_auxii:w #1 \q_nil }
\cs_new:Npn \@@_grab_c_end_auxii:w \q_nil #1 \obeyedline \q_nil
  { \@@_grab_c_end_auxiii:w #1 \q_nil }
\cs_new:Npn \@@_grab_c_end_auxiii:w #1 \q_nil #2 \q_stop
  { \exp_not:n {#1} }
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
%
% \begin{macro}
%   {
%     \@@_grab_D:w                          ,
%     \@@_grab_D_no_strip:w                 ,
%     \@@_grab_D_obey_spaces:w              ,
%     \@@_grab_D_obey_spaces_no_strip:w     ,
%     \@@_grab_D_long:w                     ,
%     \@@_grab_D_long_no_strip:w            ,
%     \@@_grab_D_long_obey_spaces:w         ,
%     \@@_grab_D_long_obey_spaces_no_strip:w
%   }
% \changes{v1.1a}{2022/08/10}{Add support for skipping brace stripping}
% \changes{v1.3a}{2025-01-23}{Auto-generate D-grabbers}
%   The generic delimited argument grabber. The auxiliary function does
%   a peek test before calling \cs{@@_grab_D_call:Nw}, so that the
%   optional nature of the argument works as expected.
%    \begin{macrocode}
\tl_map_inline:nn { { } { _long } }
  {
    \tl_map_inline:nn { { } { _obey_spaces } }
      {
        \tl_map_inline:nn { { } { _no_strip } }
          {
            \tl_map_inline:nn { { } { _verb_safe } }
              {
                \cs_new_protected:cpe { @@_grab_D #1 ##1 ####1 ########1 :w }
                  ################1################2################3
                  \@@_run_code:
                  {
                    \exp_not:N \@@_grab_D_aux:NNnNNNN
                      ################1 ################2 {################3}
                      \str_if_eq:nnTF {#1} { _long }
                        \cs_set_protected:Npn
                        \cs_set_protected_nopar:Npn
                      \str_if_eq:nnTF {##1} { _obey_spaces }
                        { \exp_not:N \@@_peek_meaning_remove:NTF }
                        { \exp_not:N \@@_peek_nonspace_remove:NTF }
                      \str_if_eq:nnTF {####1} { _no_strip }
                        { \exp_not:N \use_none:n }
                        { \exp_not:N \use_ii:nn }
                      \str_if_eq:nnTF {########1} { _verb_safe }
                        { \exp_not:N \use:n }
                        { \exp_not:N \use_none:n }
                   }
               }
          }
      }
  }
%    \end{macrocode}
% \begin{macro}{\@@_grab_D_aux:NNnNNNN}
% \changes{v1.3a}{2025-01-23}{Extend to support \texttt{c}-type grabbing}
% \changes{v1.3h}{2025-11-04}{Update for \cs{NoValue} adjustment}
% \begin{macro}{\@@_grab_D_verb_safe:NN}
% \changes{v1.3a}{2025-01-23}{New macro}
% \begin{macro}{\@@_grab_D_aux:NNnNN}
%   This is a bit complicated. The idea is that, in order to check for
%   nested optional argument tokens (\texttt{[[...]]} and so on) the
%   argument needs to be grabbed without removing any braces at all. If
%   this is not done, then cases like |[{[}]| fail. So after testing for
%   an optional argument, it is collected piece-wise. Inserting a quark
%   prevents loss of braces, and there is then a test to see if there are
%   nested delimiters to handle.
%    \begin{macrocode}
\group_begin:
  \char_set_catcode_other:N \^^M
  \cs_new_protected:Npn \@@_grab_D_aux:NNnNNNN #1#2#3#4#5#6#7
    {
      \@@_grab_D_aux:NNnNN #1#2 {#3} #4 #6
      #7
        {
          \group_begin:
            \@@_grab_D_verb_safe:NN #1#5
        }
      #5 #1
        {
          #7 { \group_end: }
          \@@_grab_D_call:Nw #1
        }
        {
          #7 { \group_end: }
          \@@_add_arg:n { \NoValue }
        }
    }
\group_end:
%    \end{macrocode}
%   The only awkwardness here is the need to preserve the catcode of the search
%   token: this is done low-level for performance reasons. Only values that
%   are realistic are included. As this does not cover spaces when they are
%   skipped that has to be covered separately.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_grab_D_verb_safe:NN #1#2
  {
    \cs_set_eq:NN \do \char_set_catcode_other:N
    \dospecials
    \char_set_catcode_other:N \^^M
    \token_if_eq_meaning:NNT #2 \@@_peek_nonspace_remove:NTF
      { \char_set_catcode_space:n { `\  } }
    \use:c
      {
        char_set_catcode_
        \if_catcode:w \c_math_toggle_token #1 math_toggle \else:
        \if_catcode:w ^ #1 math_superscript \else:
        \if_catcode:w \c_math_subscript_token #1 math_subscript \else:
        \if_catcode:w A #1 letter \else:
        other \fi: \fi: \fi: \fi:
        :N
      }
        #1
  }
%    \end{macrocode}
%   Inside the \enquote{standard} grabber, there is a test to see if the
%   grabbed argument is entirely enclosed by braces. There are a couple of
%   extra factors to allow for: the argument might be entirely empty, and
%   spaces at the start and end of the input must be retained around a brace
%   group. Also notice that a \emph{blank} argument might still contain
%   spaces. To allow for suppression of brace stripping, the business end
%   is passed here as |#5|.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_grab_D_aux:NNnNN #1#2#3#4#5
  {
    \tl_set:Nn \l_@@_signature_tl {#3}
    \exp_after:wN #4 \l_@@_fn_tl ##1 #2
      {
        \tl_if_in:nnTF {##1} {#1}
          { \@@_grab_D_nested:NNnN #1 #2 {##1} #4 }
          {
            \tl_if_blank:oTF { \use_none:n ##1 }
              { \@@_add_arg:o { \use_none:n ##1 } }
              {
                \str_if_eq:eeTF
                  { \exp_not:o { \use_none:n ##1 } }
                  { { \exp_not:o { \use_ii:nnn ##1 \q_nil } } }
                  { \@@_add_arg:o { #5 ##1 } }
                  { \@@_add_arg:o { \use_none:n ##1 } }
              }
          }
      }
  }
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
%
% \begin{macro}{\@@_grab_D_nested:NNnN}
% \begin{macro}{\@@_grab_D_nested:w}
% \begin{macro}{\l_@@_nesting_a_tl}
% \begin{macro}{\l_@@_nesting_b_tl}
% \begin{macro}{\q_@@}
%   Catching nested optional arguments means more work. The aim here is
%   to collect up each pair of optional tokens without \TeX{} helping out,
%   and without counting anything. The code above will already have
%   removed the leading opening token and a closing token, but the
%   wrong one. The aim is then to work through the material grabbed
%   so far and divide it up on each opening token, grabbing a closing
%   token to match (thus working in pairs). Once there are no opening
%   tokens, then there is a second check to see if there are any
%   opening tokens in the second part of the argument (for things
%   like |[][]|). Once everything has been found, the entire collected
%   material is added to the output as a single argument. The only tricky part
%   here is ensuring that any grabbing function that might run away is named
%   after the function currently being parsed and not after \pkg{xparse}. That
%   leads to some rather complex nesting! There is also a need to prevent the
%   loss of any braces, hence the insertion and removal of quarks along the
%   way.
%    \begin{macrocode}
\tl_new:N \l_@@_nesting_a_tl
\tl_new:N \l_@@_nesting_b_tl
\quark_new:N \q_@@
\cs_new_protected:Npn \@@_grab_D_nested:NNnN #1#2#3#4
  {
    \tl_clear:N \l_@@_nesting_a_tl
    \tl_clear:N \l_@@_nesting_b_tl
    \exp_after:wN #4 \l_@@_fn_tl ##1 #1 ##2 \q_@@ ##3 #2
      {
        \tl_put_right:No \l_@@_nesting_a_tl { \use_none:n ##1 #1 }
        \tl_put_right:No \l_@@_nesting_b_tl { \use_i:nn #2 ##3 }
        \tl_if_in:nnTF {##2} {#1}
          {
            \l_@@_fn_tl
              \q_nil ##2 \q_@@ \ERROR
          }
          {
            \tl_put_right:Ne \l_@@_nesting_a_tl
              { \@@_grab_D_nested:w \q_nil ##2 \q_stop }
            \tl_if_in:NnTF \l_@@_nesting_b_tl {#1}
              {
                \tl_set_eq:NN \l_@@_tmpa_tl \l_@@_nesting_b_tl
                \tl_clear:N \l_@@_nesting_b_tl
                \exp_after:wN \l_@@_fn_tl \exp_after:wN
                  \q_nil \l_@@_tmpa_tl \q_nil \q_@@ \ERROR
              }
              {
                \tl_put_right:No \l_@@_nesting_a_tl
                  \l_@@_nesting_b_tl
                \@@_add_arg:V \l_@@_nesting_a_tl
              }
          }
      }
    \l_@@_fn_tl #3 \q_nil \q_@@ \ERROR
  }
\cs_new:Npn \@@_grab_D_nested:w #1 \q_nil \q_stop
  { \exp_not:o { \use_none:n #1 } }
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
% \begin{macro}{\@@_grab_D_call:Nw}
%   For \texttt{D} and \texttt{R}-type arguments, to avoid losing any
%   braces, a token needs to be inserted before the argument to be grabbed.
%   If the argument runs away because the closing token is missing then this
%   inserted token shows up in the terminal. Ideally, |#1| would therefore be
%   used directly, but that is no good as it will mess up the rest of the
%   grabber. Instead, a copy of |#1| with an altered category code is used,
%   as this will look right in the terminal but will not mess up the grabber.
%   The only issue then is that the category code of |#1| is unknown. So there
%   is a quick test to ensure that the inserted token can never be matched by
%   the grabber. (This assumes that the open and close delimiters are not the
%   same character with different category codes, but that really should not
%   happen in any sensible document-level syntax.)
%   An exception is when |#1| is a control sequence token, in which case the
%   character-token treatment is no good because if hit with \cs{token_to_str:N}
%   it would add sputios tokens to the argument. In this case a different
%   branch is taken. The token inserted is then the same \meta{csname} as |#1|,
%   but with a space appended, so that the grabber don't see it as another
%   of the same delimiter.
%    \begin{macrocode}
\cs_new_protected_nopar:Npn \@@_grab_D_call:Nw #1
  {
    \token_if_eq_catcode:NNTF + #1
      {
        \exp_after:wN \exp_after:wN \exp_after:wN
          \l_@@_fn_tl \char_generate:nn { `#1 } { 11 }
      }
      {
        \@@_token_if_cs:NTF #1
          {
            \exp_after:wN \l_@@_fn_tl
            \cs:w \cs_to_str:N #1 ~ \cs_end:
          }
          {
            \exp_after:wN \l_@@_fn_tl
            \token_to_str:N #1
          }
      }
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}
%   {
%     \@@_grab_E:w, \@@_grab_E_long:w,
%     \@@_grab_E_obey_spaces:w, \@@_grab_E_long_obey_spaces:w
%   }
% \changes{v1.3h}{2025-11-04}{Update for \cs{NoValue} adjustment}
% \begin{macro}{\@@_grab_E:nnNN}
% \begin{macro}{\@@_grab_E_loop:NnN}
% \begin{macro}{\@@_grab_E_finalize:}
%   Everything here needs to point to a loop.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_grab_E:w #1#2 \@@_run_code:
  {
    \@@_grab_E:nnNN {#1} {#2}
      \cs_set_protected_nopar:Npn
      \@@_peek_nonspace_remove:NTF
  }
\cs_new_protected:Npn \@@_grab_E_long:w #1#2 \@@_run_code:
  {
    \@@_grab_E:nnNN {#1} {#2}
      \cs_set_protected:Npn
      \@@_peek_nonspace_remove:NTF
  }
\cs_new_protected:Npn \@@_grab_E_obey_spaces:w #1#2 \@@_run_code:
  {
    \@@_grab_E:nnNN {#1} {#2}
      \cs_set_protected_nopar:Npn
      \@@_peek_meaning_remove:NTF
  }
\cs_new_protected:Npn \@@_grab_E_long_obey_spaces:w #1#2 \@@_run_code:
  {
    \@@_grab_E:nnNN {#1} {#2}
      \cs_set_protected:Npn
      \@@_peek_meaning_remove:NTF
  }
%    \end{macrocode}
%   A loop is needed here to allow a random ordering of keys. These are
%   searched for one at a time, with any not found needing to be tracked:
%   they can appear later. The grabbed values are held in a property list
%   which is then turned into an ordered list to be passed back to the user.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_grab_E:nnNN #1#2#3#4
  {
    \exp_after:wN #3 \l_@@_fn_tl ##1##2##3
      {
        \prop_put:Nnn \l_@@_tmp_prop {##1} {##3}
        \@@_grab_E_loop:NnN #4 { } ##2 \q_recursion_stop
      }
    \prop_clear:N \l_@@_tmp_prop
    \tl_set:Nn \l_@@_signature_tl {#2}
    \cs_set_protected:Npn \@@_grab_E_finalize:
      {
        \tl_map_inline:nn {#1}
          {
            \prop_get:NnNF \l_@@_tmp_prop {####1} \l_@@_tmpb_tl
              { \tl_set:Nn \l_@@_tmpb_tl { \NoValue } }
            \tl_put_right:Ne \l_@@_args_tl
              { { \exp_not:V \l_@@_tmpb_tl } }
          }
        \l_@@_signature_tl \@@_run_code:
      }
    \@@_grab_E_loop:NnN #4 { } #1 \q_recursion_tail \q_recursion_stop
  }
\cs_new_protected:Npn \@@_grab_E_loop:NnN #1#2#3#4 \q_recursion_stop
  {
    \cs_if_eq:NNTF #3 \q_recursion_tail
      { \@@_grab_E_finalize: }
      {
        #1 #3
          { \l_@@_fn_tl #3 {#2#4} }
          { \@@_grab_E_loop:NnN #1 {#2#3} #4 \q_recursion_stop }
      }
  }
\cs_new_protected:Npn \@@_grab_E_finalize: { }
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
%
% \begin{macro}{\@@_grab_m:w}
% \begin{macro}{\@@_grab_m_long:w}
%   Collecting a single mandatory argument is quite easy.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_grab_m:w #1 \@@_run_code:
  {
    \tl_set:Nn \l_@@_signature_tl {#1}
    \exp_after:wN \cs_set_protected_nopar:Npn \l_@@_fn_tl ##1
      { \@@_add_arg:n {##1} }
    \l_@@_fn_tl
  }
\cs_new_protected:Npn \@@_grab_m_long:w #1 \@@_run_code:
  {
    \tl_set:Nn \l_@@_signature_tl {#1}
    \exp_after:wN \cs_set_protected:Npn \l_@@_fn_tl ##1
      { \@@_add_arg:n {##1} }
    \l_@@_fn_tl
  }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}{\@@_grab_m_1:w}
% \begin{macro}{\@@_grab_m_2:w}
% \begin{macro}{\@@_grab_m_3:w}
% \begin{macro}{\@@_grab_m_4:w}
% \begin{macro}{\@@_grab_m_5:w}
% \begin{macro}{\@@_grab_m_6:w}
% \begin{macro}{\@@_grab_m_7:w}
% \begin{macro}{\@@_grab_m_8:w}
% \begin{macro}{\@@_grab_m_9:w}
% \begin{macro}{\@@_grab_m_aux:Nnnnnnnnn}
%   Grabbing 1--8 mandatory arguments is done by giving 8--1 known
%   arguments to a 9-argument function that stores them in
%   \cs{l_@@_args_tl}.  For simplicity, grabbing 9 mandatory arguments
%   is done by grabbing 5 then 4 arguments.
%    \begin{macrocode}
\cs_new_protected_nopar:Npn \@@_grab_m_aux:Nnnnnnnnn #1#2#3#4#5#6#7#8#9
  {
    \tl_put_right:No \l_@@_args_tl
      { #1 {#2} {#3} {#4} {#5} {#6} {#7} {#8} {#9} }
    \l_@@_signature_tl \@@_run_code:
  }
\cs_new_protected:cpn { @@_grab_m_1:w } #1 \@@_run_code:
  {
    \tl_set:Nn \l_@@_signature_tl {#1}
    \exp_after:wN \cs_set_eq:NN \l_@@_fn_tl \@@_grab_m_aux:Nnnnnnnnn
    \l_@@_fn_tl \use_none:nnnnnnn { } { } { } { } { } { } { }
  }
\cs_new_protected:cpn { @@_grab_m_2:w } #1 \@@_run_code:
  {
    \tl_set:Nn \l_@@_signature_tl {#1}
    \exp_after:wN \cs_set_eq:NN \l_@@_fn_tl \@@_grab_m_aux:Nnnnnnnnn
    \l_@@_fn_tl \use_none:nnnnnn { } { } { } { } { } { }
  }
\cs_new_protected:cpn { @@_grab_m_3:w } #1 \@@_run_code:
  {
    \tl_set:Nn \l_@@_signature_tl {#1}
    \exp_after:wN \cs_set_eq:NN \l_@@_fn_tl \@@_grab_m_aux:Nnnnnnnnn
    \l_@@_fn_tl \use_none:nnnnn { } { } { } { } { }
  }
\cs_new_protected:cpn { @@_grab_m_4:w } #1 \@@_run_code:
  {
    \tl_set:Nn \l_@@_signature_tl {#1}
    \exp_after:wN \cs_set_eq:NN \l_@@_fn_tl \@@_grab_m_aux:Nnnnnnnnn
    \l_@@_fn_tl \use_none:nnnn { } { } { } { }
  }
\cs_new_protected:cpn { @@_grab_m_5:w } #1 \@@_run_code:
  {
    \tl_set:Nn \l_@@_signature_tl {#1}
    \exp_after:wN \cs_set_eq:NN \l_@@_fn_tl \@@_grab_m_aux:Nnnnnnnnn
    \l_@@_fn_tl \use_none:nnn { } { } { }
  }
\cs_new_protected:cpn { @@_grab_m_6:w } #1 \@@_run_code:
  {
    \tl_set:Nn \l_@@_signature_tl {#1}
    \exp_after:wN \cs_set_eq:NN \l_@@_fn_tl \@@_grab_m_aux:Nnnnnnnnn
    \l_@@_fn_tl \use_none:nn { } { }
  }
\cs_new_protected:cpn { @@_grab_m_7:w } #1 \@@_run_code:
  {
    \tl_set:Nn \l_@@_signature_tl {#1}
    \exp_after:wN \cs_set_eq:NN \l_@@_fn_tl \@@_grab_m_aux:Nnnnnnnnn
    \l_@@_fn_tl \use_none:n { }
  }
\cs_new_protected:cpn { @@_grab_m_8:w } #1 \@@_run_code:
  {
    \tl_set:Nn \l_@@_signature_tl {#1}
    \exp_after:wN \cs_set_eq:NN \l_@@_fn_tl \@@_grab_m_aux:Nnnnnnnnn
    \l_@@_fn_tl \prg_do_nothing:
  }
\cs_new_protected:cpe { @@_grab_m_9:w }
  {
    \exp_not:c { @@_grab_m_5:w }
    \exp_not:c { @@_grab_m_4:w }
  }
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
%
% \begin{macro}{\@@_grab_R:w, \@@_grab_R_long:w}
% \begin{macro}{\@@_grab_R_aux:NNnN}
% \changes{v1.1a}{2022/08/10}{Track changes in \texttt{D}-type implementation}
% \changes{v1.3h}{2025-11-04}{Update for \cs{NoValue} adjustment}
%  The grabber for \texttt{R}-type arguments is basically the same as
%  that for \texttt{D}-type ones, but always skips spaces (as it is mandatory)
%  and has a hard-coded error message.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_grab_R:w #1#2#3 \@@_run_code:
  { \@@_grab_R_aux:NNnN #1 #2 {#3} \cs_set_protected_nopar:Npn }
\cs_new_protected:Npn \@@_grab_R_long:w #1#2#3 \@@_run_code:
  { \@@_grab_R_aux:NNnN #1 #2 {#3} \cs_set_protected:Npn }
\cs_new_protected:Npn \@@_grab_R_aux:NNnN #1#2#3#4
  {
    \@@_grab_D_aux:NNnNN #1 #2 {#3} #4 \use_ii:nn
    \@@_peek_nonspace_remove:NTF #1
      { \@@_grab_D_call:Nw #1 }
      {
        \msg_error:nnee { cmd } { missing-required }
          { \@@_environment_or_command: }
          { \token_to_str:N #1 }
        \@@_add_arg:n { \NoValue }
      }
  }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}{\@@_grab_t:w}
% \begin{macro}{\@@_grab_t_obey_spaces:w}
% \begin{macro}{\@@_grab_t_aux:NNw}
%   Dealing with a token is quite easy. Check the match, remove the
%   token if needed and add a flag to the output.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_grab_t:w
  { \@@_grab_t_aux:NNw \@@_peek_nonspace_remove:NTF }
\cs_new_protected:Npn \@@_grab_t_obey_spaces:w
  { \@@_grab_t_aux:NNw \@@_peek_meaning_remove:NTF }
\cs_new_protected:Npn \@@_grab_t_aux:NNw #1#2#3 \@@_run_code:
  {
    \tl_set:Nn \l_@@_signature_tl {#3}
    \exp_after:wN \cs_set_protected:Npn \l_@@_fn_tl
      {
        #1 #2
          { \@@_add_arg:n { \BooleanTrue } }
          { \@@_add_arg:n { \BooleanFalse } }
      }
    \l_@@_fn_tl
  }
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \end{macro}
%
% \begin{variable}{\l_@@_v_arg_tl}
%    \begin{macrocode}
\tl_new:N \l_@@_v_arg_tl
%    \end{macrocode}
% \end{variable}
%
% \begin{macro}{\@@_grab_v:w}
% \begin{macro}{\@@_grab_v_long:w}
% \begin{macro}{\@@_grab_v_aux:w}
% \begin{macro}{\@@_grab_v_group_end:}
%   Firstly, it is necessary to change \cs{tex_endlinechar:D} so that
%   newlines in different catcode regimes (e.g., \cs{ExplSyntaxOn}) are
%   not misinterpreted as spaces.
% \changes{v1.1d}{2023/04/13}{Set \cs{tex_endlinechar:D} earlier (gh/876).}
%   The opening delimiter is the first non-space token, and is never
%   read verbatim.  This is required by consistency with the case where
%   the preceding argument was optional and absent: then \TeX{} has
%   already read and tokenized that token when looking for the optional
%   argument.  The first thing is thus to check is that this delimiter
%   is a character, and to distinguish the case of a left brace (in that
%   case, \cs{group_align_safe_end:} is needed to compensate for the
%   begin-group character that was just seen).  Then set verbatim
%   catcodes with \cs{@@_grab_v_aux_catcodes:}.
%
%   The group keep catcode changes local, and
%   \cs{group_align_safe_begin/end:} allow to use a character
%   with category code~$4$ (normally |&|) as the delimiter (all commands
%   do \cs{group_align_safe_begin/end:}, so there's no need to do that
%   again here).
%   It is ended by \cs{@@_grab_v_group_end:}, which smuggles
%   the collected argument out of the group.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_grab_v:w
  {
    \bool_set_false:N \l_@@_long_bool
    \@@_grab_v_aux:w
  }
\cs_new_protected:Npn \@@_grab_v_long:w
  {
    \bool_set_true:N \l_@@_long_bool
    \@@_grab_v_aux:w
  }
\cs_new_protected:Npn \@@_grab_v_aux:w #1 \@@_run_code:
  {
    \tl_set:Nn \l_@@_signature_tl {#1}
    \group_begin:
      \tex_escapechar:D = 92 \scan_stop:
      \tex_endlinechar:D = `\^^M \scan_stop:
      \tl_clear:N \l_@@_v_arg_tl
      \peek_remove_spaces:n
        {
          \peek_meaning_remove:NTF \c_group_begin_token
            {
              \group_align_safe_end:
              \@@_grab_v_bgroup:
            }
            {
              \peek_N_type:TF
                { \@@_grab_v_aux_test:N }
                { \@@_grab_v_aux_abort:n { } }
            }
        }
  }
\cs_new_protected:Npn \@@_grab_v_group_end:
  {
      \exp_args:NNNo
    \group_end:
    \tl_set:Nn \l_@@_v_arg_tl { \l_@@_v_arg_tl }
  }
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
%
% \begin{macro}{\@@_grab_v_aux_test:N}
% \begin{macro}
%   {
%     \@@_grab_v_aux_loop:N,
%     \@@_grab_v_aux_loop:NN,
%     \@@_grab_v_aux_loop_end:
%   }
%   Check that the opening delimiter is a character, setup category codes,
%   then start reading tokens one by one, keeping the delimiter as an argument.
%   If the verbatim was not nested, we will be grabbing one character
%   at each step. Unfortunately, it can happen that what follows the
%   verbatim argument is already tokenized. Thus, we check at each step
%   that the next token is indeed a \enquote{nice}
%   character, \emph{i.e.}, is not a character with
%   category code $1$ (begin-group), $2$ (end-group)
%   or $6$ (macro parameter), nor the space character,
%   with category code~$10$ and character code~$32$,
%   nor a control sequence.
%   The partially built argument is stored in \cs{l_@@_v_arg_tl}.
%   If we ever meet a token which we cannot grab (non-N-type),
%   or which is not a character according to
%   \cs{@@_grab_v_token_if_char:NTF}, then we bail out with
%   \cs{@@_grab_v_aux_abort:n}. Otherwise, we stop at the first
%   character matching the delimiter.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_grab_v_aux_test:N #1
  {
    \@@_grab_v_token_if_char:NTF #1
      {
        \@@_grab_v_aux_put:N #1
        \@@_grab_v_aux_catcodes:
        \@@_grab_v_aux_loop:N #1
      }
      { \@@_grab_v_aux_abort:n {#1} #1 }
  }
\cs_new_protected:Npn \@@_grab_v_aux_loop:N #1
  {
    \peek_N_type:TF
      { \@@_grab_v_aux_loop:NN #1 }
      { \@@_grab_v_aux_abort:n { } }
  }
\cs_new_protected:Npn \@@_grab_v_aux_loop:NN #1#2
  {
    \@@_grab_v_token_if_char:NTF #2
      {
        \token_if_eq_charcode:NNTF #1 #2
          { \@@_grab_v_aux_loop_end: }
          {
            \@@_grab_v_aux_put:N #2
            \@@_grab_v_aux_loop:N #1
          }
      }
      { \@@_grab_v_aux_abort:n {#2} #2 }
  }
\cs_new_protected:Npn \@@_grab_v_aux_loop_end:
  {
    \@@_grab_v_group_end:
    \@@_add_arg:e { \tl_tail:N \l_@@_v_arg_tl }
  }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{variable}{\l_@@_v_nesting_int}
%    \begin{macrocode}
\int_new:N \l_@@_v_nesting_int
%    \end{macrocode}
% \end{variable}
%
% \begin{macro}{\@@_grab_v_bgroup:}
% \begin{macro}{\@@_grab_v_bgroup_loop:}
% \begin{macro}{\@@_grab_v_bgroup_loop:N}
%   If the opening delimiter is a left brace, we keep track of
%   how many left and right braces were encountered so far in
%   \cs{l_@@_v_nesting_int} (the methods used for optional
%   arguments cannot apply here), and stop as soon as it reaches~$0$.
%
%   Some care was needed when removing the opening delimiter, which
%   has already been assigned category code~$1$: using
%   \cs{peek_meaning_remove:NTF} in the \cs{@@_grab_v_aux:w}
%   function would break within alignments. Instead, we first
%   convert that token to a string, and remove the result as a
%   normal undelimited argument.
%    \begin{macrocode}
\cs_new_protected:Npe \@@_grab_v_bgroup:
  {
    \exp_not:N \@@_grab_v_aux_catcodes:
    \exp_not:n { \int_set:Nn \l_@@_v_nesting_int { 1 } }
    \exp_not:N \@@_grab_v_aux_put:N \iow_char:N \{
    \exp_not:N \@@_grab_v_bgroup_loop:
  }
\cs_new_protected:Npn \@@_grab_v_bgroup_loop:
  {
    \peek_N_type:TF
      { \@@_grab_v_bgroup_loop:N }
      { \@@_grab_v_aux_abort:n { } }
  }
\cs_new_protected:Npn \@@_grab_v_bgroup_loop:N #1
  {
    \@@_grab_v_token_if_char:NTF #1
      {
        \token_if_eq_charcode:NNTF \c_group_end_token #1
          {
            \int_decr:N \l_@@_v_nesting_int
            \int_compare:nNnTF \l_@@_v_nesting_int > 0
              {
                \@@_grab_v_aux_put:N #1
                \@@_grab_v_bgroup_loop:
              }
              { \@@_grab_v_aux_loop_end: }
          }
          {
            \token_if_eq_charcode:NNT \c_group_begin_token #1
              { \int_incr:N \l_@@_v_nesting_int }
            \@@_grab_v_aux_put:N #1
            \@@_grab_v_bgroup_loop:
          }
      }
      { \@@_grab_v_aux_abort:n {#1} #1 }
  }
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \end{macro}
%
% \begin{macro}{\@@_grab_v_aux_catcodes:}
% \changes{v1.2g}{2025-01-21}{Store spaces and tabs as active chars}
% \begin{macro}{\@@_grab_v_aux_abort:n}
% \changes{v1.3h}{2025-11-04}{Update for \cs{NoValue} adjustment}
%   The approach for short verbatim arguments is to make the end-line
%   character a macro parameter character: this is forbidden by the
%   rest of the code. Then the error branch can check what caused the
%   bail out and give the appropriate error message.
%    \begin{macrocode}
%<latexrelease>\IncludeInRelease{2025/06/01}{\@@_grab_v_aux_catcodes:}%
%<latexrelease>  {Active~spaces~and~tabs}
\cs_new_protected:Npn \@@_grab_v_aux_catcodes:
  {
    \cs_set_eq:NN \do \char_set_catcode_other:N
    \dospecials
    \char_set_catcode_active:n { `\  }
    \char_set_catcode_active:n { `\^^I }
    \bool_if:NTF \l_@@_long_bool
      { \char_set_catcode_other:n { \tex_endlinechar:D } }
      { \char_set_catcode_parameter:n { \tex_endlinechar:D } }
  }
%<latexrelease>\EndIncludeInRelease
%<latexrelease>\IncludeInRelease{2020/10/01}{\@@_grab_v_aux_catcodes:}%
%<latexrelease>  {Active~spaces~and~tabs}
%<latexrelease>\cs_new_protected:Npn \@@_grab_v_aux_catcodes:
%<latexrelease>  {
%<latexrelease>    \cs_set_eq:NN \do \char_set_catcode_other:N
%<latexrelease>    \dospecials
%<latexrelease>    \bool_if:NTF \l_@@_long_bool
%<latexrelease>      { \char_set_catcode_other:n { \tex_endlinechar:D } }
%<latexrelease>      { \char_set_catcode_parameter:n { \tex_endlinechar:D } }
%<latexrelease>  }
%<latexrelease>\EndIncludeInRelease
\cs_new_protected:Npn \@@_grab_v_aux_abort:n #1
  {
    \@@_grab_v_group_end:
    \exp_after:wN \exp_after:wN \exp_after:wN
      \peek_meaning_remove:NTF \char_generate:nn { \tex_endlinechar:D } { 6 }
      {
        \msg_error:nneee { cmd } { verbatim-nl }
          { \@@_environment_or_command: }
          { \tl_to_str:N \l_@@_v_arg_tl }
          { \tl_to_str:n {#1} }
        \@@_add_arg:n { \NoValue }
      }
      {
        \msg_error:nneee { cmd } { verbatim-tokenized }
          { \@@_environment_or_command: }
          { \tl_to_str:N \l_@@_v_arg_tl }
          { \tl_to_str:n {#1} }
        \@@_add_arg:n { \NoValue }
      }
  }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}{\@@_grab_v_aux_put:N}
% \changes{v1.2d}{2024/03/21}{Collect \cs{endlinechar} as \cs{obeyedline}}
% \changes{v1.2g}{2025-01-21}{Simplify catcode handling}
%   Storing one token in the collected argument: everything as-is except
%   for end-of-lines, with \cs{exp_not:N} to handle actives.
%    \begin{macrocode}
%<latexrelease>\IncludeInRelease{2025-06-01}{\@@_grab_v_aux_put:N}%
%<latexrelease>  {Use~more~std~catcodes}
\cs_new_protected:Npn \@@_grab_v_aux_put:N #1
  {
    \tl_put_right:Ne \l_@@_v_arg_tl
      {
        \int_compare:nNnTF {`#1} = \tex_endlinechar:D
          { \exp_not:N \obeyedline }
          { \exp_not:N #1 }
      }
  }
%<latexrelease>\EndIncludeInRelease
%<latexrelease>\IncludeInRelease{2024/06/01}{\@@_grab_v_aux_put:N}%
%<latexrelease>  {Endlines~as~\obeyedline}
%<latexrelease>\cs_new_protected:Npn \@@_grab_v_aux_put:N #1
%<latexrelease>  {
%<latexrelease>    \tl_put_right:Nx \l_@@_v_arg_tl
%<latexrelease>      {
%<latexrelease>        \token_if_active:NTF #1
%<latexrelease>          { \exp_not:N #1 }
%<latexrelease>          {
%<latexrelease>            \int_compare:nNnTF {`#1} = \tex_endlinechar:D
%<latexrelease>              { \exp_not:N \obeyedline }
%<latexrelease>              { \token_to_str:N #1 }
%<latexrelease>          }
%<latexrelease>      }
%<latexrelease>  }
%<latexrelease>\EndIncludeInRelease
%<latexrelease>\IncludeInRelease{2020/10/01}{\@@_grab_v_aux_put:N}%
%<latexrelease>  {Endlines~as~\obeyedline}
%<latexrelease>\cs_new_protected:Npn \@@_grab_v_aux_put:N #1
%<latexrelease>  {
%<latexrelease>    \tl_put_right:Nx \l_@@_v_arg_tl
%<latexrelease>      {
%<latexrelease>        \token_if_active:NTF #1
%<latexrelease>          { \exp_not:N #1 } { \token_to_str:N #1 }
%<latexrelease>      }
%<latexrelease>  }
%<latexrelease>\EndIncludeInRelease
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\@@_grab_v_token_if_char:NTF}
%   This function assumes that the escape character is printable.
%   Then the string representation of control sequences is at least
%   two characters, and \cs{str_tail:n} only removes the escape
%   character. Macro parameter characters are doubled by
%   \cs{tl_to_str:n}, and will also yield a non-empty result,
%   hence are not considered as characters.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_grab_v_token_if_char:NTF #1
  { \str_if_eq:eeTF { } { \str_tail:n {#1} } }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\@@_add_arg:n, \@@_add_arg:V, \@@_add_arg:o, \@@_add_arg:e}
%   When an argument is found it is stored, then further arguments are
%   grabbed by calling \cs{l_@@_signature_tl}.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_add_arg:n #1
  {
    \tl_put_right:Nn \l_@@_args_tl { {#1} }
    \l_@@_signature_tl \@@_run_code:
  }
\cs_generate_variant:Nn \@@_add_arg:n { V , o , e }
%    \end{macrocode}
% \end{macro}
%
% \subsection{Grabbing arguments expandably}
%
% \begin{macro}[EXP]{\@@_expandable_grab_D:w}
% \begin{macro}[EXP]{\@@_expandable_grab_D:NNNwNNn}
% \changes{v1.3h}{2025-11-04}{Update for \cs{NoValue} adjustment}
% \begin{macro}[EXP]{\@@_expandable_grab_D:NNNwNNnnn}
% \begin{macro}[EXP]{\@@_expandable_grab_D:Nw}
% \begin{macro}[EXP]{\@@_expandable_grab_D:nnNNNwNN}
%   The first step is to grab the first token or group. The generic grabbers
%   \cs{\meta{function}}\verb*| | and \cs{\meta{function}}\verb*| | are just after \cs{q_@@}, we go and find
%   them (and use the long one).
%    \begin{macrocode}
\cs_new:Npn \@@_expandable_grab_D:w #1 \q_@@ #2#3
  { #2 { \@@_expandable_grab_D:NNNwNNn #1 \q_@@ #2 #3 } }
%    \end{macrocode}
%   We then wish to test whether |#7|, which we just grabbed, is exactly |#2|.
%   A preliminary test is whether their string representations coincide, then
%   expand the only grabber function we have, |#1|, once: the two strings below
%   are equal if and only if |#7| matches |#2| exactly.\footnote{It is obvious
%   that if \texttt{\#7} matches \texttt{\#2} then the strings are equal. We
%   must check the converse. The right-hand-side of \cs{str_if_eq:onTF} does
%   not end with \texttt{\#3}, implying that the grabber function took
%   everything as its arguments. The first brace group can only be empty if
%   \texttt{\#7} starts with \texttt{\#2}, otherwise the brace group preceding
%   \texttt{\#7} would not vanish. The third brace group is empty, thus the
%   \cs{q_@@} that was used by our grabber \texttt{\#1} must be the one
%   that we inserted (not some token in \texttt{\#7}), hence the second brace
%   group contains the end of \texttt{\#7} followed by \texttt{\#2}. Since this
%   is \texttt{\#2} on the right-hand-side, and no brace can be lost there,
%   \texttt{\#7} must contain nothing else than its leading \texttt{\#2}.}
%   The preliminary test is needed as |#7| could validly contain
%   \tn{par} (because a later mandatory argument could be long) and our
%   grabber may be short.  If
%   |#7| does not match |#2|, then the optional argument is missing, we use the
%   default \cs{NoValue}, and put back the argument |#7| in the input stream.
%
%   If it does match, then interesting things need to be done. We will grab the
%   argument piece by piece, with the following pattern:
%   \begin{quote}
%     \meta{grabber} \Arg{tokens} \\
%     ~~\cs{q_nil} \Arg{piece 1} \meta{piece 2} \cs{ERROR} \cs{q_@@}\\
%     ~~\cs{q_nil} \meta{input stream}
%   \end{quote}
%   The \meta{grabber} will find an opening delimiter in \meta{piece 2}, take
%   the \cs{q_@@} as a second delimiter, and find more material delimited
%   by the closing delimiter in the \meta{input stream}. We then move the part
%   before the opening delimiter from \meta{piece 2} to \meta{piece 1}, and the
%   material taken from the \meta{input stream} to the \meta{piece 2}. Thus,
%   the argument moves gradually from the \meta{input stream} to the
%   \meta{piece 2}, then to the \meta{piece 1} when we have made sure to find
%   all opening and closing delimiters. This two-step process ensures that
%   nesting works: the number of opening delimiters minus closing delimiters in
%   \meta{piece 1} is always equal to the number of closing delimiters in
%   \meta{piece 2}. We stop grabbing arguments once the \meta{piece 2} contains
%   no opening delimiter any more, hence the balance is reached, and the final
%   argument is \meta{piece 1} \meta{piece 2}.
%    \begin{macrocode}
\cs_new:Npn \@@_expandable_grab_D:NNNwNNn #1#2#3#4 \q_@@ #5#6#7
  {
    \str_if_eq:nnTF {#2} {#7}
      {
        \str_if_eq:onTF
          { #1 { } { } #7 #2 \q_@@ #3 }
          { { } {#2} { } }
      }
      { \use_ii:nn }
      {
        #1
          { \@@_expandable_grab_D:NNNwNNnnn #1#2#3#4 \q_@@ #5#6 }
          \q_nil { } #2 \ERROR \q_@@ \ERROR
      }
      { #4 { \NoValue } \q_@@ #5 #6 {#7} }
  }
%    \end{macrocode}
%   At this stage, |#7| is \cs{q_nil} \Arg{piece 1} \meta{more for piece 1},
%   and we want to concatenate all that, removing \cs{q_nil}, and keeping the
%   opening delimiter |#2|. Simply use \cs{use_ii:nn}. Also, |#8| is
%   \meta{remainder of piece 2} \cs{ERROR}, and |#9| is \cs{ERROR} \meta{more
%   for piece 2}. We concatenate those, replacing the two \cs{ERROR} by the
%   closing delimiter |#3|.
%    \begin{macrocode}
\cs_new:Npn \@@_expandable_grab_D:NNNwNNnnn #1#2#3#4 \q_@@ #5#6#7#8#9
  {
    \exp_args:Nof \@@_expandable_grab_D:nnNNNwNN
      { \use_ii:nn #7 #2 }
      { \@@_expandable_grab_D:Nw #3 \exp_stop_f: #8 #9 }
    #1#2#3 #4 \q_@@ #5 #6
  }
\cs_new:Npn \@@_expandable_grab_D:Nw #1#2 \ERROR \ERROR { #2 #1 }
%    \end{macrocode}
%   Armed with our two new \meta{pieces}, we are ready to loop. However, we
%   must first see if \meta{piece 2} (here |#2|) contains any opening
%   delimiter |#4|. Again, we expand |#3|, this time removing its whole output
%   with \cs{use_none:nnn}. The test is similar to \cs{tl_if_in:nnTF}. The
%   token list is empty if and only if |#2| does not contain the opening
%   delimiter. In that case, we are done, and put the argument (from which we
%   remove a spurious pair of delimiters coming from how we started the loop).
%   Otherwise, we go back to looping with
%   \cs{@@_expandable_grab_D:NNNwNNnnn}. The code to deal with brace stripping
%   is much the same as for the non-expandable case.
%    \begin{macrocode}
\cs_new:Npn \@@_expandable_grab_D:nnNNNwNN #1#2#3#4#5#6 \q_@@ #7#8
  {
    \exp_args:No \tl_if_empty:oTF
      { #3 { \use_none:nnn } #2 \q_@@ #5 #4 \q_@@ #5 }
      {
        \tl_if_blank:oTF { \use_none:nn #1#2 }
          { \@@_put_arg_expandable:ow { \use_none:nn #1#2 } }
          {
            \str_if_eq:eeTF
              { \exp_not:o { \use_none:nn #1#2 } }
              { { \exp_not:o { \use_iii:nnnn #1#2 \q_nil } } }
              { \@@_put_arg_expandable:ow { \use_iii:nnn #1#2 } }
              { \@@_put_arg_expandable:ow { \use_none:nn #1#2 } }
          }
            #6 \q_@@ #7 #8
      }
      {
        #3
          { \@@_expandable_grab_D:NNNwNNnnn #3#4#5#6 \q_@@ #7 #8 }
          \q_nil {#1} #2 \ERROR \q_@@ \ERROR
      }
  }
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
%
% \begin{macro}[EXP]{\@@_expandable_grab_D_alt:w}
% \begin{macro}[EXP]{\@@_expandable_grab_D_alt:NNwNNn}
% \changes{v1.3h}{2025-11-04}{Update for \cs{NoValue} adjustment}
% \begin{macro}[EXP]{\@@_expandable_grab_D_alt:Nwn}
%   When the delimiters are identical, nesting is not possible and a simplified
%   approach is used. The test concept here is the same as for the case where
%   the delimiters are different but there cannot be any nesting.
%    \begin{macrocode}
\cs_new:Npn \@@_expandable_grab_D_alt:w #1 \q_@@ #2#3
  { #2 { \@@_expandable_grab_D_alt:NNwNNn #1 \q_@@ #2 #3 } }
\cs_new:Npn \@@_expandable_grab_D_alt:NNwNNn #1#2#3 \q_@@ #4#5#6
  {
    \str_if_eq:nnTF {#6} {#2}
      {
        \str_if_eq:onTF
          { #1 { } #6 #2 #2 }
          { { } #2 }
      }
      { \use_ii:nn }
      {
        #1
          { \@@_expandable_grab_D_alt:NNwn #4 #5 #3 \q_@@ }
          #6 \ERROR
      }
      { #3 { \NoValue } \q_@@ #4 #5 {#6} }
  }
\cs_new:Npn \@@_expandable_grab_D_alt:NNwn #1#2#3 \q_@@ #4
  {
    \tl_if_blank:oTF { \use_none:n #4 }
      { \@@_put_arg_expandable:ow { \use_none:n #4 } }
      {
        \str_if_eq:eeTF
          { \exp_not:o { \use_none:n #4 } }
          { { \exp_not:o { \use_ii:nnn #4 \q_nil } } }
          { \@@_put_arg_expandable:ow { \use_ii:nn #4 } }
          { \@@_put_arg_expandable:ow { \use_none:n #4 } }
      }
        #3 \q_@@ #1 #2
  }
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \end{macro}
%
% \begin{macro}[EXP]{\@@_expandable_grab_E:w, \@@_expandable_grab_E_long:w}
% \begin{macro}[EXP]{\@@_expandable_grab_E_aux:w}
% \begin{macro}[EXP]{\@@_expandable_grab_E_test:nnw}
% \begin{macro}[EXP]{\@@_expandable_grab_E_loop:nnnNNw}
% \begin{macro}[EXP]{\@@_expandable_grab_E_find:w}
% \begin{macro}[EXP]{\@@_expandable_grab_E_find:nnw}
% \begin{macro}[EXP]{\@@_expandable_grab_E_end:nnw}
%   We keep track of long/short by placing the appropriate grabber as
%   the third token after \cs{q_@@}; it is eventually removed by the
%   \texttt{end:nnw} auxiliary.  The \texttt{aux:w} auxiliary will be
%   called repeatedly with two arguments: the set
%   of pairs \meta{parser} \meta{token}, and the set of arguments found
%   so far (initially all |{\NoValue}|).  At each step, grab what
%   follows in the input stream then call the \texttt{loop:nnnNNw}
%   auxiliary to compare it with each possible embellishment in turn.
%   This auxiliary's |#1| is what was found in the input, |#2| collects
%   \meta{parser} \meta{token} pairs that did not match, |#3| collects
%   the corresponding arguments found previously, |#4| and |#5| is the
%   current pair, |#6| is the remaining pairs, |#7| is empty or two
%   \cs{q_nil}, and |#8| is the current argument.  If none of the pairs
%   matched (determined by \cs{quark_if_nil:NTF}) then call the
%   \texttt{end} auxiliary to stop looking for embellishments,
%   remembering to put what was grabbed in the input back where it
%   belongs, and storing the arguments found just before \cs{q_@@}.  If
%   the current argument |#8| is not \cs{NoValue} or if the input |#1|
%   does not match |#5| (see \texttt{t}-type arguments below for a
%   similar \cs{str_if_eq:onTF} test) then carry on the loop.
%   Otherwise, we found a new embellishment: grab the corresponding
%   argument in the input using the \texttt{find:w} auxiliary.  To avoid
%   losing braces around that auxiliary's argument we include a
%   space, which will be eliminated in the next loop through
%   embellishments.
%    \begin{macrocode}
\cs_new:Npn \@@_expandable_grab_E:w #1 \q_@@ #2#3
  { \@@_expandable_grab_E_aux:w #1 \q_@@ #2 #3 #3 }
\cs_new:Npn \@@_expandable_grab_E_long:w #1 \q_@@ #2#3
  { \@@_expandable_grab_E_aux:w #1 \q_@@ #2 #3 #2 }
\cs_new:Npn \@@_expandable_grab_E_aux:w #1 \q_@@ #2#3#4
  { #2 { \@@_expandable_grab_E_test:nnw #1 \q_@@ #2 #3 #4 } }
\cs_new:Npn \@@_expandable_grab_E_test:nnw #1#2#3 \q_@@ #4#5#6#7
  {
    \@@_expandable_grab_E_loop:nnnNNw {#7} { } { }
      #1 \q_nil \q_nil \q_nil \q_mark #2 \q_nil
    #3 \q_@@ #4 #5 #6
  }
\cs_new:Npn \@@_expandable_grab_E_loop:nnnNNw
    #1#2#3#4#5#6 \q_nil #7 \q_mark #8
  {
    \quark_if_nil:NTF #4
      { \@@_expandable_grab_E_end:nnw {#1} {#3} }
      {
        \tl_if_novalue:nTF {#8}
          { \str_if_eq:onTF { #4 { } #1 #5 } {#5} }
          { \use_ii:nn }
            { \@@_expandable_grab_E_find:w { #2 #4 #5 #6 } {#3} ~ }
            {
              \@@_expandable_grab_E_loop:nnnNNw
                {#1} { #2 #4 #5 } { #3 {#8} }
                #6 \q_nil #7 \q_mark
            }
      }
  }
\cs_new:Npn \@@_expandable_grab_E_find:w #1 \q_@@ #2#3#4
  { #4 { \@@_expandable_grab_E_find:nnw #1 \q_@@ #2 #3 #4 } }
\cs_new:Npn \@@_expandable_grab_E_find:nnw #1#2#3 \q_nil #4 \q_@@ #5#6#7#8
  { \@@_expandable_grab_E_aux:w {#1} { #2 {#8} #3 } #4 \q_@@ #5 #6 #7 }
\cs_new:Npn \@@_expandable_grab_E_end:nnw #1#2#3 \q_@@ #4#5#6
  { #3 #2 \q_@@ #4 #5 {#1} }
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
%
% \begin{macro}[EXP]{\@@_expandable_grab_m:w, \@@_expandable_grab_m_long:w}
% \begin{macro}[EXP]{\@@_expandable_grab_m_aux:wNn}
%   The mandatory case is easy: find the auxiliary after the \cs{q_@@},
%   and use it directly to grab the argument, then correctly position
%   the argument before \cs{q_@@}.
%    \begin{macrocode}
\cs_new:Npn \@@_expandable_grab_m:w #1 \q_@@ #2#3
  { #3 { \@@_expandable_grab_m_aux:wNn #1 \q_@@ #2 #3 } }
\cs_new:Npn \@@_expandable_grab_m_long:w #1 \q_@@ #2#3
  { #2 { \@@_expandable_grab_m_aux:wNn #1 \q_@@ #2 #3 } }
\cs_new:Npn \@@_expandable_grab_m_aux:wNn #1 \q_@@ #2#3#4
  { #1 {#4} \q_@@ #2 #3 }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}[EXP]{\@@_expandable_grab_R:w}
% \begin{macro}[EXP]{\@@_expandable_grab_R_aux:NNNwNNn}
% \changes{v1.3h}{2025-11-04}{Update for \cs{NoValue} adjustment}
%   Much the same as for the \texttt{D}-type argument, with only the lead-off
%   function varying.
%    \begin{macrocode}
\cs_new:Npn \@@_expandable_grab_R:w #1 \q_@@ #2#3
  { #2 { \@@_expandable_grab_R_aux:NNNwNNn #1 \q_@@ #2#3 } }
\cs_new:Npn \@@_expandable_grab_R_aux:NNNwNNn #1#2#3#4 \q_@@ #5#6#7
  {
    \str_if_eq:nnTF {#7} {#2}
      {
        \str_if_eq:onTF
          { #1 { } { } #7 #2 \q_@@ #3 }
          { { } {#2} { } }
      }
      { \use_ii:nn }
      {
        #1
          { \@@_expandable_grab_D:NNNwNNnnn #1#2#3#4 \q_@@ #5#6 }
          \q_nil { } #2 \ERROR \q_@@ \ERROR
      }
      {
        \msg_expandable_error:nnff { cmd } { missing-required }
          { \exp_args:Nf \tl_trim_spaces:n { \token_to_str:N #5 } }
          { \tl_to_str:n {#2} }
        #4 { \NoValue } \q_@@ #5 #6 {#7}
      }
  }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}[EXP]{\@@_expandable_grab_R_alt:w}
% \begin{macro}[EXP]{\@@_expandable_grab_R_alt_aux:NNwNNn}
% \changes{v1.3h}{2025-11-04}{Update for \cs{NoValue} adjustment}
%   When the delimiters are identical, nesting is not possible and a simplified
%   approach is used. The test concept here is the same as for the case where
%   the delimiters are different.
%    \begin{macrocode}
\cs_new:Npn \@@_expandable_grab_R_alt:w #1 \q_@@ #2#3
  { #2 { \@@_expandable_grab_R_alt_aux:NNwNNn #1 \q_@@ #2#3 } }
\cs_new:Npn \@@_expandable_grab_R_alt_aux:NNwNNn #1#2#3 \q_@@ #4#5#6
  {
    \str_if_eq:nnTF {#6} {#2}
      {
        \str_if_eq:onTF
          { #1 { } #6 #2 #2 }
          { { } #2 }
      }
      { \use_ii:nn }
      {
        #1
          { \@@_expandable_grab_D_alt:NNwn #4 #5 #3 \q_@@ }
          #6 \ERROR
      }
      {
        \msg_expandable_error:nnff { cmd } { missing-required }
          { \exp_args:Nf \tl_trim_spaces:n { \token_to_str:N #4 } }
          { \tl_to_str:n {#2} }
        #3 { \NoValue } \q_@@ #4 #5 {#6}
      }
  }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}[EXP]{\@@_expandable_grab_t:w}
% \begin{macro}[EXP]{\@@_expandable_grab_t_aux:NNwn}
%   As for a \texttt{D}-type argument, here we compare the grabbed tokens using
%   the only parser we have in order to work out if |#2| is exactly equal to
%   the output of the grabber.
%    \begin{macrocode}
\cs_new:Npn \@@_expandable_grab_t:w #1 \q_@@ #2#3
  { #2 { \@@_expandable_grab_t_aux:NNwn #1 \q_@@ #2 #3 } }
\cs_new:Npn \@@_expandable_grab_t_aux:NNwn #1#2#3 \q_@@ #4#5#6
  {
    \str_if_eq:onTF { #1 { } #6 #2 } {#2}
      { #3 { \BooleanTrue } \q_@@ #4 #5 }
      { #3 { \BooleanFalse } \q_@@ #4 #5 {#6} }
  }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}[EXP]
%   {\@@_put_arg_expandable:nw, \@@_put_arg_expandable:ow}
%   A useful helper, to store arguments when they are ready.
%    \begin{macrocode}
\cs_new:Npn \@@_put_arg_expandable:nw #1#2 \q_@@ { #2 {#1} \q_@@ }
\cs_generate_variant:Nn \@@_put_arg_expandable:nw { o }
%    \end{macrocode}
% \end{macro}
%
% \subsection{Argument processors}
%
% \begin{macro}{\@@_bool_reverse:N}
%   A simple reversal.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_bool_reverse:N #1
  {
    \bool_if:NTF #1
      { \tl_set:Nn \ProcessedArgument { \c_false_bool } }
      { \tl_set:Nn \ProcessedArgument { \c_true_bool } }
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{variable}{\l_@@_split_list_seq, \l_@@_split_list_tl}
% \begin{macro}{\@@_split_list:nn}
% \begin{macro}{\@@_split_list_multi:nn, \@@_split_list_multi:nV}
% \begin{macro}{\@@_split_list_single:Nn}
%   Splitting can take place either at a single token or at a longer
%   identifier. To deal with single active tokens, a two-part procedure is
%   needed.
%    \begin{macrocode}
\seq_new:N \l_@@_split_list_seq
\tl_new:N \l_@@_split_list_tl
\cs_new_protected:Npn \@@_split_list:nn #1#2
  {
    \tl_if_single:nTF {#1}
      {
        \token_if_cs:NTF #1
          { \@@_split_list_multi:nn {#1} {#2} }
          { \@@_split_list_single:Nn #1 {#2} }
      }
      { \@@_split_list_multi:nn {#1} {#2} }
  }
\cs_new_protected:Npn \@@_split_list_multi:nn #1#2
  {
    \seq_set_split:Nnn \l_@@_split_list_seq {#1} {#2}
    \tl_clear:N \ProcessedArgument
    \seq_map_inline:Nn \l_@@_split_list_seq
      { \tl_put_right:Nn \ProcessedArgument { {##1} } }
  }
\cs_generate_variant:Nn \@@_split_list_multi:nn { nV }
\group_begin:
\char_set_catcode_active:N \^^@
\cs_new_protected:Npn \@@_split_list_single:Nn #1#2
  {
    \tl_set:Nn \l_@@_split_list_tl {#2}
    \group_begin:
    \char_set_lccode:nn { `\^^@ } { `#1 }
    \tex_lowercase:D
      {
        \group_end:
        \tl_replace_all:Nnn \l_@@_split_list_tl { ^^@ }
      }   {#1}
     \@@_split_list_multi:nV {#1} \l_@@_split_list_tl
   }
\group_end:
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{variable}
%
% \begin{macro}{\@@_split_argument:nnn}
% \begin{macro}{\@@_split_argument_aux:nnnn}
% \changes{v1.3h}{2025-11-04}{Update for \cs{NoValue} adjustment}
% \begin{macro}[EXP]{\@@_split_argument_aux:n}
% \begin{macro}[rEXP]{\@@_split_argument_aux:wn}
%   Splitting to a known number of items is a special version of splitting
%   a list, in which the limit is hard-coded and where there will always be
%   exactly the correct number of output items. An auxiliary function is
%   used to save on working out the token list length several times.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_split_argument:nnn #1#2#3
  {
    \@@_split_list:nn {#2} {#3}
    \exp_args:Nf \@@_split_argument_aux:nnnn
      { \tl_count:N \ProcessedArgument }
      {#1} {#2} {#3}
  }
\cs_new_protected:Npn \@@_split_argument_aux:nnnn #1#2#3#4
  {
    \int_compare:nNnF {#1} = { #2 + 1 }
      {
        \int_compare:nNnTF {#1} > { #2 + 1 }
          {
            \tl_set:Ne \ProcessedArgument
              {
                \exp_last_unbraced:NnNo
                  \@@_split_argument_aux:n
                  { #2 + 1 }
                  \use_none_delimit_by_q_stop:w
                  \ProcessedArgument
                  \q_stop
              }
            \msg_error:nneee { cmd } { arg-split }
              { \tl_to_str:n {#3} } { \int_eval:n { #2 + 1 } }
              { \tl_to_str:n {#4} }
          }
          {
            \tl_put_right:Ne \ProcessedArgument
              {
                \prg_replicate:nn { #2 + 1 - (#1) }
                  { { \NoValue } }
              }
          }
      }
  }
%    \end{macrocode}
%   Auxiliaries to leave exactly the correct number of arguments in
%   \cs{ProcessedArgument}.
%    \begin{macrocode}
\cs_new:Npn \@@_split_argument_aux:n #1
  { \prg_replicate:nn {#1} { \@@_split_argument_aux:wn } }
\cs_new:Npn \@@_split_argument_aux:wn #1 \use_none_delimit_by_q_stop:w #2
  {
    \exp_not:n { {#2} }
    #1
    \use_none_delimit_by_q_stop:w
  }
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
%
% \begin{macro}{\@@_trim_spaces:n}
%   This one is almost trivial.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_trim_spaces:n #1
  { \tl_set:Ne \ProcessedArgument { \tl_trim_spaces:n {#1} } }
%    \end{macrocode}
% \end{macro}
%
% \subsection{Conversion to key--value form}
%
% This is implemented as a process but with no public interfaces,
% hence is treated separately from the others: it's a feature of
% \pkg{ltcmd} which just happens to use the same mechanism as a processor.
%
% \begin{macro}{\@@_arg_to_keyvalue:nn}
% \changes{v1.1a}{2022/08/10}{New internal arg-to-keyval processor}
% \begin{macro}{\@@_arg_to_keyvalue_braces:nnn}
% \changes{v1.3i}{2026-02-02}{Treat empty input as empty keyval list}
% \begin{macro}{\@@_arg_to_keyvalue_auxi:nnn}
% \begin{macro}{\@@_arg_to_keyvalue_auxii:Nnnn}
% \begin{macro}{\@@_arg_to_keyvalue_auxiii:nnn}
% \begin{macro}{\@@_arg_to_keyvalue_auxiv:Nnnn}
% \begin{macro}{\@@_arg_to_keyvalue_auxv:nn}
% \begin{macro}{\@@_arg_to_keyvalue_loop:w}
% \begin{macro}{\@@_arg_to_keyvalue_loop_group:n}
% \begin{macro}{\@@_arg_to_keyvalue_loop_space:w}
% \begin{macro}{\@@_arg_to_keyvalue_loop_N_type:N}
% \begin{macro}{\@@_arg_to_keyvalue_math:w}
% \begin{macro}{\@@_arg_to_keyvalue_math_N_type:N}
% \begin{macro}{\@@_arg_to_keyvalue_math_group:n}
% \begin{macro}{\@@_arg_to_keyvalue_math_space:w}
% \begin{macro}{\@@_arg_to_keyvalue_set_default:nn}
% \begin{macro}{\@@_arg_to_keyvalue_set_keyvalue:nn}
% \begin{macro}[EXP]{\@@_split_N_head_apply:Nn}
% \begin{macro}[EXP]{\@@_split_N_head_apply_aux:NNw}
%   If the entire argument is braced, we treat as free text and return as
%   the value for the text key. Alternatively, if the start of the input is
%   |=,| then it is forced to be key--value. To avoid needing to worry about
%   catcodes for this, and to allow spaces around the |=|, we use a
%   series of steps rather than a delimited argument. We also deal with the
%   case where the entire argument is blank (or is an entirely empty brace
%   group).
%     \begin{macrocode}
\cs_new_protected:Npn \@@_arg_to_keyvalue:nn #1#2
  {
    \tl_trim_spaces_apply:nN {#2} \@@_arg_to_keyvalue_braces:nnn
      {#1} {#2}
  }
\cs_new_protected:Npn \@@_arg_to_keyvalue_braces:nnn #1#2#3
  {
    \bool_lazy_and:nnTF
      { \tl_if_head_is_group_p:n {#1} }
      { \tl_if_blank_p:o { \use_none:n #1 } }
      { \tl_set:Ne \ProcessedArgument { #2 = { \exp_not:n #1 } } }
      {
        \tl_if_blank:nTF {#1}
          { \tl_clear:N \ProcessedArgument }
          { \@@_arg_to_keyvalue_auxi:nnn {#1} {#2} {#3} }
      }
  }
\cs_new:Npn \@@_arg_to_keyvalue_auxi:nnn #1
  {
    \tl_if_head_is_N_type:nTF {#1}
      { \@@_split_N_head_apply:Nn \@@_arg_to_keyvalue_auxii:Nnnn {#1} }
      { \@@_arg_to_keyvalue_auxv:nn }
  }
\cs_new:Npn \@@_arg_to_keyvalue_auxii:Nnnn #1#2
  {
    \str_if_eq:eeTF { \exp_not:n {#1} } { = }
      { \tl_trim_spaces_apply:nN {#2} \@@_arg_to_keyvalue_auxiii:nnn }
      { \@@_arg_to_keyvalue_auxv:nn }
  }
\cs_new:Npn \@@_arg_to_keyvalue_auxiii:nnn #1
  {
    \tl_if_head_is_N_type:nTF {#1}
      { \@@_split_N_head_apply:Nn \@@_arg_to_keyvalue_auxiv:Nnnn {#1} }
      { \@@_arg_to_keyvalue_auxv:nn }
  }
\cs_new:Npn \@@_arg_to_keyvalue_auxiv:Nnnn #1#2
  {
    \str_if_eq:eeTF { \exp_not:n {#1} } { , }
      { \tl_set:Nn \ProcessedArgument {#2} \use_none:nn }
      { \@@_arg_to_keyvalue_auxv:nn }
  }
%    \end{macrocode}
%   The two clear-cut cases have been eliminated, and we therefore have to deal
%   with a search for |=| signs. We need an \enquote{action} loop here
%   so we do not get mislead by for example |{=}|. As the code here is for
%   very much predictable types of input, we hard-code what constitutes
%   math mode opening and closing.  At the very beginning, the default
%   key (|#1|) and the argument as given by the user (|#2|) are placed
%   right after the \cs{q_@@_recursion_stop}, so that when the recursion
%   ends, the macros \cs{@@_arg_to_keyvalue_set_default:nn} or
%   \cs{@@_arg_to_keyvalue_set_keyvalue:nn} can be used to grab these
%   two items and set the \cs{ProcessedArgument} accordingly.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_arg_to_keyvalue_auxv:nn #1#2
  {
    \@@_arg_to_keyvalue_loop:w #2
      \q_@@_recursion_tail \q_@@_recursion_stop {#1} {#2}
  }
\cs_new_protected:Npn \@@_arg_to_keyvalue_loop:w #1 \q_@@_recursion_stop
  {
    \tl_if_head_is_N_type:nTF {#1}
      { \@@_arg_to_keyvalue_loop_N_type:N }
      {
        \tl_if_head_is_group:nTF {#1}
          { \@@_arg_to_keyvalue_loop_group:n }
          { \@@_arg_to_keyvalue_loop_space:w }
      }
        #1 \q_@@_recursion_stop
  }
\cs_new_protected:Npn \@@_arg_to_keyvalue_loop_group:n #1
  { \@@_arg_to_keyvalue_loop:w }
\use:n { \cs_new_protected:Npn \@@_arg_to_keyvalue_loop_space:w } ~
  { \@@_arg_to_keyvalue_loop:w }
\cs_new_protected:Npn \@@_arg_to_keyvalue_loop_N_type:N #1
  {
    \@@_if_recursion_tail_stop_do:Nn #1
      { \@@_arg_to_keyvalue_set_default:nn }
    \str_if_eq:nnTF {#1} { = }
      {
        \@@_use_i_delimit_by_q_recursion_stop:nw
          { \@@_arg_to_keyvalue_set_keyvalue:nn }
      }
      {
        \bool_lazy_or:nnTF
            { \token_if_math_toggle_p:N #1 }
            { \str_if_eq_p:nn {#1} { \( } }
          { \@@_arg_to_keyvalue_math:w }
          { \@@_arg_to_keyvalue_loop:w }
      }
  }
\cs_new_protected:Npn \@@_arg_to_keyvalue_math:w #1 \q_@@_recursion_stop
  {
    \tl_if_head_is_N_type:nTF {#1}
      { \@@_arg_to_keyvalue_math_N_type:N }
      {
        \tl_if_head_is_group:nTF {#1}
          { \@@_arg_to_keyvalue_math_group:n }
          { \@@_arg_to_keyvalue_math_space:w }
      }
        #1 \q_@@_recursion_stop
  }
\cs_new_protected:Npn \@@_arg_to_keyvalue_math_N_type:N #1
  {
    \@@_if_recursion_tail_stop_do:Nn #1
      { \@@_arg_to_keyvalue_set_default:nn }
    \bool_lazy_or:nnTF
        { \token_if_math_toggle_p:N #1 }
        { \str_if_eq_p:nn {#1} { \) } }
      { \@@_arg_to_keyvalue_loop:w }
      { \@@_arg_to_keyvalue_math:w }
  }
\cs_new_protected:Npn \@@_arg_to_keyvalue_math_group:n #1
  { \@@_arg_to_keyvalue_math:w }
\use:n { \cs_new_protected:Npn \@@_arg_to_keyvalue_math_space:w } ~
  { \@@_arg_to_keyvalue_math:w }
\cs_new_protected:Npn \@@_arg_to_keyvalue_set_default:nn #1#2
  { \tl_set:Nn \ProcessedArgument { #1 = {#2} } }
\cs_new_protected:Npn \@@_arg_to_keyvalue_set_keyvalue:nn #1#2
  { \tl_set:Nn \ProcessedArgument {#2} }
%    \end{macrocode}
%   A utility to allow us to grab the first \texttt{N}-type token without
%   risking brace stripping the rest of the input.
%    \begin{macrocode}
\cs_new:Npn \@@_split_N_head_apply:Nn #1#2
  { \exp:w \if_false: { \fi: \@@_split_N_head_apply_aux:NNw #1#2 } }
\cs_new:Npn \@@_split_N_head_apply_aux:NNw #1#2
  {
    \exp_after:wN \exp_end:
    \exp_after:wN #1 \exp_after:wN #2 \exp_after:wN { \if_false: } \fi:
  }

%    \end{macrocode}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
%
% \subsection{Utilities}
%
% \begin{macro}{\@@_check_definable:nNT, \@@_check_definable_aux:nN}
%   Check that a token list is appropriate as a first argument of
%   \cs{NewDocumentCommand} and similar functions and otherwise
%   produce an error.  First trim whitespace to allow for spaces around
%   the actual command to be defined.  If the result has multiple
%   tokens, it is not a valid argument.  The single token is a control
%   sequence exactly if its string representation has more than one
%   character (using \cs{token_to_str:N} rather than \cs{tl_to_str:n}
%   to avoid problems with macro parameter characters, and setting
%   \cs{tex_escapechar:D} to prevent it from being non-printable).
%   Finally, check for an active character: this is done by lowercasing
%   the token to fix its character code (arbitrarily to that of~|?|)
%   and comparing the result to an active~|?|.  Both control sequences
%   and active characters are valid arguments, and non-active character
%   tokens are not.  In all cases, the group opened to keep assignments
%   local must be closed.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_check_definable:nNT #1
  { \tl_trim_spaces_apply:nN {#1} \@@_check_definable_aux:nN }
\group_begin:
  \char_set_catcode_active:n { `? }
  \cs_new_protected:Npn \@@_check_definable_aux:nN #1#2
    {
      \group_begin:
      \tl_if_single_token:nTF {#1}
        {
          \int_set:Nn \tex_escapechar:D { 92 }
          \exp_args:Ne \tl_if_empty:nTF
            { \exp_args:No \str_tail:n { \token_to_str:N #1 } }
            {
              \exp_args:Ne \char_set_lccode:nn
                { ` \str_head:n {#1} } { `? }
              \tex_lowercase:D { \tl_if_eq:nnTF {#1} } { ? }
                { \group_end: \use_iii:nnn }
                { \group_end: \use_i:nnn }
            }
            { \group_end: \use_iii:nnn }
        }
        { \group_end: \use_ii:nnn }
      {
        \msg_error:nnee { cmd } { not-definable }
          { \tl_to_str:n {#1} } { \token_to_str:N #2 }
      }
      {
        \msg_error:nnee { cmd } { not-one-token }
          { \tl_to_str:n {#1} } { \token_to_str:N #2 }
      }
    }
\group_end:
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\@@_token_if_cs:NTF}
%   Based on the definition of \cs{@@_check_definable_aux:nN} above, but
%   only checks for an actual control sequence (\emph{i.e.},
%   \cs[no-index]{\meta{anything}}). \cs{tex_escapechar:D} is
%   temporarily changed to a known value and then it checks if
%   |\string#1| contains more than one character: if it does, it's a
%   control sequence.  This test differs from \cs{token_if_cs:NTF} for
%   example in \verb|\token_if_cs:NTF \c_group_begin_token {T}{F}|,
%   where \cs{token_if_cs:NTF} returns false.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_token_if_cs:NTF #1
  {
    \group_begin:
      \int_set:Nn \tex_escapechar:D { 92 }
      \exp_args:Ne \tl_if_empty:nTF
          { \exp_args:No \str_tail:n { \token_to_str:N #1 } }
        { \group_end: \use_ii:nn }
        { \group_end: \use_i:nn }
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\@@_tl_mapthread_function:NNN, \@@_tl_mapthread_function:nnN}
% \begin{macro}{\@@_tl_mapthread_loop:w}
%   Analogue of \cs{seq_mapthread_function:NNN} for token lists.
%    \begin{macrocode}
\cs_new:Npn \@@_tl_mapthread_function:NNN #1#2#3
  {
    \exp_after:wN \exp_after:wN
    \exp_after:wN \@@_tl_mapthread_loop:w
    \exp_after:wN \exp_after:wN
    \exp_after:wN #3
    \exp_after:wN #1
    \exp_after:wN \q_recursion_tail
    \exp_after:wN \q_mark
    #2
    \q_recursion_tail
    \q_recursion_stop
  }
\cs_new:Npn \@@_tl_mapthread_function:nnN #1#2#3
  {
    \@@_tl_mapthread_loop:w #3
      #1 \q_recursion_tail \q_mark
      #2 \q_recursion_tail \q_recursion_stop
  }
\cs_new:Npn \@@_tl_mapthread_loop:w #1#2#3 \q_mark #4
  {
    \quark_if_recursion_tail_stop:n {#2}
    \quark_if_recursion_tail_stop:n {#4}
    #1 {#2} {#4}
    \@@_tl_mapthread_loop:w #1#3 \q_mark
  }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}{\__kernel_cmd_if_xparse:NTF}
% \changes{v1.3g}{2025/10/02}{Remove protected status}
% \begin{macro}{\@@_cmd_type_cases:NnnnnnF}
% \changes{v1.0d}{2021/04/19}{Renamed \cs{__cmd_cmd_if_xparse:NTF} to
%      \cs{__kernel_cmd_if_xparse:NTF} for cross-module usage}
% \changes{v1.0d}{2021/07/30}{Added \cs{__cmd_cmd_type_cases:NnnnnF} for
%      \cs{NewCommandCopy} and \cs{ShowCommand} support}
% \changes{v1.0l}{2022/03/18}{Fix \cs{__cmd_cmd_type_cases:NnnnnF}
%      prematurely expanding macros (gh/795)}
% \changes{v1.2b}{2023/12/01}{Extend for optimized commands}
% \changes{v1.2e}{2024/04/17}{Use \cs{__kernel_cs_parameter_spec:N} instead
%   of \cs{cs_argument_spec:N}/\cs{cs_parameter_spec:N}}
% \changes{v1.3g}{2025/10/02}{Remove protected status}
% \begin{macro}{\@@_cmd_if_xparse_aux:N}
%
%    To determine whether the command is an \pkg{xparse} command check
%    that its |arg_spec| is empty (this also excludes non-macros) and
%    that its |replacement_spec| starts with either
%    \cs{@@_start:nNNnnn} (non-expandable command) or
%    \cs{@@_start_expandable:nNNNNn} (expandable command) or
%    \cs{@@_start_optimized:} (optimized command) or
%    \cs{@@_start_env:nnnnn} (environment) or
%    \cs[no-index]{environment~\#1~end~aux} (environment end).
%
%    This conditional is needed in several kernel modules and is
%    therefore has a kernel-internal name.
%    \begin{macrocode}
\cs_new:Npn \@@_cmd_type_cases:NnnnnnF #1 #2 #3 #4 #5 #6 #7
  {
    \exp_args:Ne \str_case_e:nnF
      {
        \exp_args:Nf \tl_if_empty:nT { \__kernel_cs_parameter_spec:N #1 }
          { \exp_not:N \exp_not:n { \exp_not:e { \tl_head:N #1 } } }
      }
      {
        { \exp_not:N \@@_start:nNNnnn } {#2}
        { \exp_not:N \@@_start_expandable:nNNNNn } {#3}
        { \exp_not:N \@@_start_optimized: } {#4}
        { \exp_not:N \@@_start_env:nnnnn } {#5}
        {
          \exp_after:wN \exp_not:N
            \cs:w environment~
              \exp_last_unbraced:Ne \use_none:nnn
                { \cs_to_str:N #1 } ~end~aux \cs_end:
        } {#6}
      }
      {#7}
  }
\cs_new:Npn \__kernel_cmd_if_xparse:NTF #1
  {
    \@@_cmd_type_cases:NnnnnnF #1
      { } { } { } { } { } { \use_iii:nnn }
    \use_i:nn
  }
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \end{macro}
%
% \begin{macro}{\@@_peek_nonspace:NTF, \@@_peek_nonspace_remove:NTF, \@@_peek_nonspace_aux:nNNTF}
%   Collect spaces in a loop, and put the collected spaces back in the
%   false branch of a call to \cs{peek_meaning:NTF} or
%   \cs{peek_meaning_remove:NTF}.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_peek_nonspace:NTF
  { \@@_peek_nonspace_aux:nNNTF { } \@@_peek_meaning:NTF }
\cs_new_protected:Npn \@@_peek_nonspace_remove:NTF
  { \@@_peek_nonspace_aux:nNNTF { } \@@_peek_meaning_remove:NTF }
\cs_new_protected:Npn \@@_peek_nonspace_aux:nNNTF #1#2#3#4#5
  {
    \peek_meaning_remove:NTF \c_space_token
      { \@@_peek_nonspace_aux:nNNTF { #1 ~ } #2 #3 {#4} {#5} }
      { #2 #3 { #4 } { #5 #1 } }
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\@@_peek_meaning:NTF, \@@_peek_meaning_remove:NTF}
% \begin{macro}{\@@_peek_cs_check_equal:NNN, \@@_peek_meaning_aux:NNTF, \@@_peek_true_remove:NNw}
%   Peek ahead for a token with a given meaning.  In case the search
%   token is a control sequence, also check that the \meta{csname} is
%   the same as the control sequence peeked at.  This extra verification
%   is necessary when the command is delimited by control sequence tokens
%   (as opposed to character tokens), and we want the exact same
%   control sequence to match.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_peek_meaning:NTF
  { \@@_peek_meaning_aux:NNTF \c_false_bool }
\cs_new_protected:Npn \@@_peek_meaning_remove:NTF
  { \@@_peek_meaning_aux:NNTF \c_true_bool }
\cs_new_protected:Npn \@@_peek_meaning_aux:NNTF #1#2#3#4
  {
    \tl_set:Nn \l_@@_tmpa_tl {#3}
    \tl_set:Nn \l_@@_tmpb_tl {#4}
    \peek_meaning:NTF #2
      {
        \token_if_eq_meaning:NNTF #2 \c_group_begin_token
          { \@@_peek_true_remove:Nw #1 }
          {
            \@@_token_if_cs:NTF #2
              { \@@_peek_cs_check_equal:NNN #1 #2 }
              { \@@_peek_true_remove:Nw #1 }
          }
      }
      { \l_@@_tmpb_tl }
  }
\cs_new_protected:Npn \@@_peek_cs_check_equal:NNN #1#2#3
  {
    \str_if_eq:nnTF {#2} {#3}
      { \@@_peek_true_remove:Nw #1 }
      { \l_@@_tmpb_tl }
    #3
  }
\cs_new_protected:Npn \@@_peek_true_remove:Nw #1
  {
    \bool_if:NTF #1
      {
        \tex_afterassignment:D \l_@@_tmpa_tl
        \cs_set_eq:NN \@@_tmp:w
      }
      { \l_@@_tmpa_tl }
  }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \subsection{Access to the argument specification}
%
% \begin{macro}[EXP]{\cmd_arg_spec:N, \cmd_arg_spec:c}
% \changes{v1.3g}{2025/10/02}{New function}
% \begin{macro}[EXP]{\@@_arg_spec_opt:N}
% \changes{v1.3g}{2025/10/02}{New function}
%   First we check that the command passed does exist, and is one with the
%   correct form. It if does then there are two cases. For optimised commands,
%   we can reconstruct the arg.~spec.\ from the \TeX{} parameter spec, taking
%   care to add |+| if required. For the non-optimized cases, the
%   arg.~spec.\ is stored in the top-level macro. We treat this as a |tl| and
%   extract the appropriate balanced text: the second item.
%    \begin{macrocode}
\cs_new:Npn \cmd_arg_spec:N #1
  {
    \cs_if_exist:NTF #1
      {
        \__kernel_cmd_if_xparse:NTF #1
          {
            \exp_args:No \tl_if_head_eq_meaning:nNTF #1
              \@@_start_optimized:
              {
                \exp_args:Nc \@@_arg_spec_opt:N
                  { \cs_to_str:N #1 \c_space_tl code }
              }
              { \tl_item:Nn #1 { 2 } }
          }
          { X }
      }
      { X }
  }
\cs_generate_variant:Nn \cmd_arg_spec:N { c }
\cs_new:Npn \@@_arg_spec_opt:N #1
  {
    \prg_replicate:nn { \str_count:e { \cs_parameter_spec:N #1 } / 2 }
      {
        \bool_lazy_or:nnT
          { \token_if_long_macro_p:N #1 }
          { \token_if_protected_long_macro_p:N #1 }
          { + }
        m
      }
  }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \subsection{Messages}
%
% \begin{variable}{\c_@@_ignore_def_tl}
%    \begin{macrocode}
\tl_const:Nn \c_@@_ignore_def_tl
  { \\ \\ LaTeX~will~ignore~this~entire~definition. }
%    \end{macrocode}
% \end{variable}
%
% \begin{macro}{\@@_environment_or_command:}
%   Two texts used in several messages.
%    \begin{macrocode}
\cs_new:Npn \@@_environment_or_command:
  {
    \bool_if:NTF \l_@@_environment_bool
      { environment ~ ' \l_@@_environment_str ' }
      {
        command ~
          ' \c_backslash_str \tl_to_str:N \l_@@_function_tl '
      }
  }
%    \end{macrocode}
% \end{macro}
%
% Some messages intended as errors when defining commands/environments.
% \changes{v1.0f}{2021/06/04}{Normalize various error messages}
% \changes{v1.2c}{2023/12/22}
%         {Generalize message \texttt{invalid-bang} (gh/1198)}
% \changes{v1.3a}{2025-01-23}{Generalize message \texttt{arg-after-body}}
%    \begin{macrocode}
\msg_new:nnnn { cmd } { arg-after-body }
  { Argument~type~'#1'~must~be~last~in~#2. }
  {
    The~'#1'~argument~type~must~come~last~but~it~is~followed~
    by~'#3'~in~the~argument~specification.~This~is~not~allowed.
    \c_@@_ignore_def_tl
  }
\msg_new:nnnn { cmd } { bad-arg-spec }
  { Bad~argument~specification~'#2'~for~#1. }
  {
    The~argument~specification~provided~is~not~valid:~
    one~or~more~mandatory~parts~are~missing.
    \c_@@_ignore_def_tl
  }
\msg_new:nnnn { cmd } { already-defined }
  { Command~'#1'~already~defined. }
  {
    You~have~used~#2~
    with~a~command~that~already~has~a~definition. \\ \\
    The~existing~definition~of~'#1'~will~not~be~altered.
  }
\msg_new:nnnn { cmd } { undefined }
  { Command ~'#1'~undefined. }
  {
    You~have~used~#2~
    with~a~command~that~was~never~defined.
    \c_@@_ignore_def_tl
  }
%    \end{macrocode}
% \changes{v1.3a}{2025-01-23}{New message \texttt{chars-dropped-first-line}}
% \changes{v1.3a}{2025-01-23}{New message \texttt{chars-dropped-last-line}}
%    \begin{macrocode}
\msg_new:nnnn { cmd } { chars-dropped-first-line }
  { Characters~'#1'~dropped~on~first~line~of~#2~environment. }
  {
    LaTeX~was~collecting~a~verbatim-like~environment,~and~the~characters~
    '#1'~were~found~after~'\begin{#2}~on~the~first~line:~this~is~not~supported.
  }
\msg_new:nnnn { cmd } { chars-dropped-last-line }
  { Characters~'#1'~dropped~after~end~of~#2~environment. }
  {
    LaTeX~was~collecting~a~verbatim-like~environment,~and~the~characters~
    '#1'~were~found~after~\end{#2}~on~the~last~line:~this~is~not~supported.
  }
\msg_new:nnnn { cmd } { env-already-defined }
  { Environment~'#1'~already~defined. }
  {
    You~have~used~\NewDocumentEnvironment
    with~an~environment~that~already~has~a~definition. \\ \\
    The~existing~definition~of~'#1'~will~not~be~altered.
  }
\msg_new:nnnn { cmd } { env-end-already-defined }
  { End~of~environment~'#1'~already~defined. }
  {
    You~have~used~\NewDocumentEnvironment
    with~an~environment~that~already~has~a~definition~for~'end#1'. \\ \\
    The~existing~definition~of~'#1'~will~not~be~altered.
  }
\msg_new:nnnn { cmd } { env-undefined }
  { Environment~'#1'~undefined. }
  {
    You~have~used~\RenewDocumentEnvironment
    with~an~environment~that~was~never~defined.
    \c_@@_ignore_def_tl
  }
\msg_new:nnnn { cmd } { expandable-ending-optional }
  { Bad~argument~specification~'#2'~for~#1. }
  {
    Expandable~commands~must~have~a~final~mandatory~argument~
    (or~no~arguments~at~all).~You~cannot~have~a~terminal~optional~
    argument~with~expandable~commands.
  }
\msg_new:nnnn { cmd } { long-short-mix }
  { Invalid~argument~prefix~'+'~in~command~'#1'. }
  {
    The~arguments~for~an~expandable~command~must~not~involve~short~
    arguments~after~long~arguments.~You~have~tried~to~mix~the~two~types~
    when~defining~'#1'.
  }
\msg_new:nnnn { cmd } { invalid-command-arg }
  { Invalid~argument~type~'#2'~in~#1. }
  {
    The~letter~'#2'~can~only~be~used~in~environment~argument~
    specifications,~but~not~for~commands.
    \\ \\
    LaTeX~will~ignore~the~entire~definition.
  }
\msg_new:nnnn { cmd } { invalid-expandable-arg }
  { Invalid~argument~type~'#2'~in~#1. }
  {
    The~letter~'#2'~specifies~an~argument~type~which~cannot~be~used~
    in~an~expandable~command.
    \c_@@_ignore_def_tl
  }
\msg_new:nnnn { cmd } { invalid-after-optional-expandably }
  { Argument~'#2'~invalid~after~optional~arg~in~#1. }
  {
    The~letter~'#2'~specifies~an~argument~type~which~cannot~be~used~
    in~an~expandable~command~after~an~optional~argument.
    \c_@@_ignore_def_tl
  }
\msg_new:nnnn { cmd } { invalid-bang }
  { Invalid~argument~prefix~'!'~in~#1. }
  {
    The~prefix~'!'~is~only~allowed~for~trailing~optional~arguments.~
    You~tried~to~apply~it~to~#2.
    \c_@@_ignore_def_tl
  }
\msg_new:nnnn { cmd } { not-definable }
  { First~argument~of~'#2'~must~be~a~command. }
  {
    The~first~argument~of~'#2'~should~be~the~document~command~that~will~
    be~defined.~The~provided~argument~'#1'~is~a~character.~Perhaps~a~
    backslash~is~missing?
    \c_@@_ignore_def_tl
  }
\msg_new:nnnn { cmd } { not-one-token }
  { First~argument~of~'#2'~must~be~a~command. }
  {
    The~first~argument~of~'#2'~should~be~the~document~command~that~will~
    be~defined.~The~provided~argument~'#1'~contains~more~than~one~
    token.~Perhaps~a~backslash~is~missing?
    \c_@@_ignore_def_tl
  }
\msg_new:nnnn { cmd } { not-single-token }
  { Argument~delimiter~'#2'~invalid~in~#1. }
  {
    The~argument~specification~contains~
    \tl_if_empty:nTF{#2}{nothing}{'#2'}~
    in~a~place~
    where~a~single~token~is~required.
    \c_@@_ignore_def_tl
  }
\msg_new:nnnn { cmd } { forbidden-group-token }
  { Argument~delimiter~'#2'~invalid~in~#1. }
  {
    The~argument~specification~contains~the~implicit~
    #3-group~token~'#2'~which~is~not~allowed~as~an~argument~delimiter.
    \c_@@_ignore_def_tl
  }
\msg_new:nnnn { cmd } { processor-in-expandable }
  { Invalid~argument~prefix~'>'~in~command~'#1'. }
  {
    The~argument~specification~for~'#1'~contains~the~processor~function~'>{#2}'.~
    This~is~only~supported~for~robust~commands,~but~not~for~expandable~ones.
    \c_@@_ignore_def_tl
  }
\msg_new:nnnn { cmd } { keyval-in-expandable }
  { Invalid~argument~prefix~'='~in~command~'#1'. }
  {
    The~argument~specification~for~'#1'~contains~a~key--value~marker~'={#2}'.~
    This~is~only~supported~for~robust~commands,~but~not~for~expandable~ones.
    \c_@@_ignore_def_tl
  }
\msg_new:nnnn { cmd } { too-many-args }
  { Too~many~arguments~for~#1. }
  {
    The~argument~specification~'#2'~asks~for~more~than~9~arguments.~
    This~cannot~be~implemented.
    \c_@@_ignore_def_tl
  }
\msg_new:nnnn { cmd } { two-markers }
  { Invalid~argument~prefix~'#2'~in~#1. }
  {
    The~argument~specification~provided~for~#1~has~two~'#2'~markers~applied~
    to~the~same~argument;~one~is~redundant.
  }
\msg_new:nnnn { cmd } { unknown-argument-type } % should be unkown-arg-type but dep in xparse
  { Invalid~argument~type~'#2'~in~#1. }
  {
    The~letter~'#2'~does~not~specify~a~known~argument~type.
    \c_@@_ignore_def_tl
  }
\msg_new:nnnn { cmd } { xparse-arg-type }
  { Invalid~argument~type~'#2'~in~#1~(requires~xparse). }
  {
    The~letter~'#2'~specifies~a~known~but~deprecated~argument~type.~
    If~you~really~need~it~you~have~to~load~the~xparse~package.
    \c_@@_ignore_def_tl
  }
%    \end{macrocode}
%
% Errors when using commands/environments.  The \texttt{if-boolean}
% message is always used in expandable errors.  The
% \texttt{default-loop} and \texttt{missing-required} messages can
% be expandable or not expandable.
%    \begin{macrocode}
\msg_new:nnn { cmd } { if-boolean }
  { Invalid~argument~{#1}~to~\iow_char:N\\IfBoolean... }
\msg_new:nnnn { cmd } { default-loop }
  { Circular~dependency~in~defaults~of~#1. }
  {
    The~default~values~of~two~or~more~arguments~of~the~#1~
    depend~on~each~other~in~a~way~that~cannot~be~resolved.
  }
\msg_new:nnnn { cmd } { missing-required }
  { Required~argument~missing~for~#1. }
  {
    The~#1~expects~one~of~its~arguments~to~start~with~'#2'.~
    LaTeX~did~not~find~this~argument~and~will~insert~a~default~value~
    for~further~processing.
  }
\msg_new:nnnn { cmd } { arg-split }
  { Too~many~'#1'~separators~in~argument. }
  {
    LaTeX~was~asked~to~split~the~input~'#3'~
    at~each~occurrence~of~the~separator~'#1'~into~#2~parts.~
    Too~many~separators~were~found.
  }
\msg_new:nnnn { cmd } { verbatim-nl }
  { Verbatim-like~#1~ended~by~end~of~line. }
  {
    The~verbatim~argument~of~the~#1~cannot~contain~more~than~one~line,~
    but~the~end~
    of~the~current~line~has~been~reached.~You~may~have~forgotten~the~
    closing~delimiter.
    \\ \\
    LaTeX~will~ignore~'#2'~and~you~may~get~some~additional~
    (low-level)~errors.
  }
\msg_new:nnnn { cmd } { verbatim-tokenized }
  { Verbatim-like~#1~illegal~in~argument. }
  {
    The~#1~takes~a~verbatim~argument~and~should~therefore~normally~
    not~be~used~in~arguments~of~other~commands~or~environments.~
    LaTeX~found~an~illegal~token~ \tl_if_empty:nF {#3} { (#3)~ }
    after~'#2'~and~will~drop~everything~up~to~this~point.
    \\ \\
    Expect~further~(low-level)~errors.
  }
%    \end{macrocode}
%
% Intended more for information.
%    \begin{macrocode}
\msg_new:nnn { cmd } { define-command }    % should be just `define' but dep in xparse
  {
    Defining~command~#1~
    with~sig.~'#2'~\msg_line_context:.
  }
\msg_new:nnn { cmd } { define-env }
  {
    Defining~environment~'#1'~
    with~sig.~'#2'~\msg_line_context:.
  }
\msg_new:nnn { cmd } { redefine }
  {
    Redefining~command~#1~
    with~sig.~'#2'~\msg_line_context:.
  }
\msg_new:nnn { cmd } { redefine-env }
  {
    Redefining~environment~'#1'~
    with~sig.~'#2'~\msg_line_context:.
  }
\msg_new:nnn { cmd } { optional-mandatory }
  {
    Optional~and~mandatory~argument~with~same~delimiter~'#2'.
    \\ \\
    The~mandatory~argument~specified~with~
    '\str_case:nnF{#1}{ {R/r}{r'~or~'R} }{#1}'~has~the~
    same~delimiter~'#2'~as~an~earlier~optional~argument.~
    It~will~therefore~not~be~possible~to~omit~all~the~earlier~
    optional~arguments~when~calling~this~command.
    \\ \\
    This~may~be~intentional,~but~then~it~might~be~a~mistake.
  }
\msg_new:nnn { cmd } { unsupported-let }
  {
    The~command~'#1'~was~undefined~but~not~the~associated~commands~
    '#1~code'~and/or~'#1~defaults'.~Maybe~you~tried~using~
    \iow_char:N\\let.~This~may~lead~to~an~infinite~loop.
  }
%    \end{macrocode}
%
% \subsection{User functions}
%
% The user functions are more or less just the internal functions
% renamed.
%
% \begin{macro}{\BooleanFalse}
% \begin{macro}{\BooleanTrue}
%   Design-space names for the Boolean values.
%    \begin{macrocode}
\cs_new_eq:NN \BooleanFalse \c_false_bool
\cs_new_eq:NN \BooleanTrue  \c_true_bool
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}{\NewDocumentCommand}
% \begin{macro}{\RenewDocumentCommand}
% \begin{macro}{\ProvideDocumentCommand}
% \begin{macro}{\DeclareDocumentCommand}
%   The user macros are pretty simple wrappers around the internal ones.
%   There is however a check that the first argument is a single token,
%   possibly surrounded by spaces (hence the strange \cs{use:nnn}), and
%   is definable.
%    \begin{macrocode}
\cs_new_protected:Npn \NewDocumentCommand #1#2#3
  {
    \@@_check_definable:nNT {#1} \NewDocumentCommand
      {
        \cs_if_exist:NTF #1
          {
            \msg_error:nnee { cmd } { already-defined }
              { \use:nnn \token_to_str:N #1 { } }
              { \token_to_str:N \NewDocumentCommand }
          }
          { \@@_declare_cmd:Nnn #1 {#2} {#3} }
      }
  }
\cs_new_protected:Npn \RenewDocumentCommand #1#2#3
  {
    \@@_check_definable:nNT {#1} \RenewDocumentCommand
      {
        \cs_if_exist:NTF #1
          { \@@_declare_cmd:Nnn #1 {#2} {#3} }
          {
            \msg_error:nnee { cmd } { undefined }
              { \use:nnn \token_to_str:N #1 { } }
              { \token_to_str:N \RenewDocumentCommand }
          }
      }
  }
\cs_new_protected:Npn \ProvideDocumentCommand #1#2#3
  {
    \@@_check_definable:nNT {#1} \ProvideDocumentCommand
      { \cs_if_exist:NF #1 { \@@_declare_cmd:Nnn #1 {#2} {#3} } }
 }
\cs_new_protected:Npn \DeclareDocumentCommand #1#2#3
  {
    \@@_check_definable:nNT {#1} \DeclareDocumentCommand
      { \@@_declare_cmd:Nnn #1 {#2} {#3} }
  }
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
%
% \begin{macro}{\NewDocumentEnvironment}
% \changes{v1.0h}{2021/08/27}{Check for end-of-environment command}
% \changes{v1.2f}{2024/07/06}{Trim spaces from envname first (gh/1399)}
% \begin{macro}{\RenewDocumentEnvironment}
% \begin{macro}{\ProvideDocumentEnvironment}
% \begin{macro}{\DeclareDocumentEnvironment}
% \begin{macro}{\@@_new_env:nnnn, \@@_renew_env:nnnn, \@@_provide_env:nnnn}
% \begin{macro}{\@@_new_env:ennn, \@@_renew_env:ennn, \@@_provide_env:ennn}
%   Very similar for environments. Trim spaces from user-specified
%   \meta{envname}, do existence check then hand off to
%   \cs{@@_declare_env:nnnn}.
%    \begin{macrocode}
%<latexrelease>\IncludeInRelease{2024/11/01}{\NewDocumentEnvironment}%
%<latexrelease>                 {Trim~spaces~from~envname~first}
\cs_new_protected:Npn \NewDocumentEnvironment #1#2#3#4
  {
    \@@_new_env:ennn { \tl_trim_spaces:e {#1} } {#2} {#3} {#4}
  }
\cs_new_protected:Npn \RenewDocumentEnvironment #1#2#3#4
  {
    \@@_renew_env:ennn { \tl_trim_spaces:e {#1} } {#2} {#3} {#4}
  }
\cs_new_protected:Npn \ProvideDocumentEnvironment #1#2#3#4
  {
    \@@_provide_env:ennn { \tl_trim_spaces:e {#1} } {#2} {#3} {#4}
  }
\cs_new_protected:Npn \DeclareDocumentEnvironment #1#2#3#4
  {
    \@@_declare_env:ennn { \tl_trim_spaces:e {#1} } {#2} {#3} {#4}
  }
%    \end{macrocode}
%   Each of \cs[no-index]{@@_(new|renew|provide)_env:nnnn} is curried.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_new_env:nnnn #1
  {
    \cs_if_exist:cTF {#1}
      {
        \msg_error:nne { cmd } { env-already-defined } {#1}
        \use_none:nnn
      }
      {
        \cs_if_exist:cTF { end #1 }
          {
            \msg_error:nne { cmd } { env-end-already-defined } {#1}
            \use_none:nnn
          }
          { \@@_declare_env:nnnn {#1} }
      }
  }
\cs_new_protected:Npn \@@_renew_env:nnnn #1
  {
    \cs_if_exist:cTF {#1}
      { \@@_declare_env:nnnn {#1} }
      {
        \msg_error:nne { cmd } { env-undefined } {#1}
        \use_none:nnn
      }
  }
\cs_new_protected:Npn \@@_provide_env:nnnn #1
  {
    \cs_if_exist:cTF {#1}
      { \use_none:nnn }
      { \@@_declare_env:nnnn {#1} }
  }
\cs_generate_variant:Nn \@@_new_env:nnnn { e }
\cs_generate_variant:Nn \@@_renew_env:nnnn { e }
\cs_generate_variant:Nn \@@_provide_env:nnnn { e }
%<latexrelease>\EndIncludeInRelease
%<latexrelease>\IncludeInRelease{2024/06/01}{\NewDocumentEnvironment}%
%<latexrelease>                 {Trim~spaces~from~envname~first}
%<latexrelease>\cs_new_protected:Npn \NewDocumentEnvironment #1#2#3#4
%<latexrelease>  {
%<latexrelease>    \cs_if_exist:cTF {#1}
%<latexrelease>      { \msg_error:nnx { cmd } { env-already-defined } {#1} }
%<latexrelease>      {
%<latexrelease>        \cs_if_exist:cTF { end #1 }
%<latexrelease>          { \msg_error:nnx { cmd } { env-end-already-defined } {#1} }
%<latexrelease>          { \@@_declare_env:nnnn {#1} {#2} {#3} {#4} }
%<latexrelease>      }
%<latexrelease>  }
%<latexrelease>\cs_new_protected:Npn \RenewDocumentEnvironment #1#2#3#4
%<latexrelease>  {
%<latexrelease>    \cs_if_exist:cTF {#1}
%<latexrelease>      { \@@_declare_env:nnnn {#1} {#2} {#3} {#4} }
%<latexrelease>      { \msg_error:nnx { cmd } { env-undefined } {#1} }
%<latexrelease>  }
%<latexrelease>\cs_new_protected:Npn \ProvideDocumentEnvironment #1#2#3#4
%<latexrelease>  { \cs_if_exist:cF {#1} { \@@_declare_env:nnnn {#1} {#2} {#3} {#4} } }
%<latexrelease>\cs_new_protected:Npn \DeclareDocumentEnvironment #1#2#3#4
%<latexrelease>  { \@@_declare_env:nnnn {#1} {#2} {#3} {#4} }
%<latexrelease>\cs_undefine:N \@@_new_env:nnnn
%<latexrelease>\cs_undefine:N \@@_new_env:ennn
%<latexrelease>\cs_undefine:N \@@_renew_env:nnnn
%<latexrelease>\cs_undefine:N \@@_renew_env:ennn
%<latexrelease>\cs_undefine:N \@@_provide_env:nnnn
%<latexrelease>\cs_undefine:N \@@_provide_env:ennn
%<latexrelease>
%<latexrelease>\EndIncludeInRelease
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
%
% \begin{macro}{\NewExpandableDocumentCommand}
% \begin{macro}{\RenewExpandableDocumentCommand}
% \begin{macro}{\ProvideExpandableDocumentCommand}
% \begin{macro}{\DeclareExpandableDocumentCommand}
%   The expandable versions are essentially the same as the basic
%   functions.  The strange \cs{use:nnn} is there in case |#1| is
%   surrounded with spaces, as can happen with usual document catcodes
%   in \cs{RenewExpandableDocumentCommand} |{| |\!| |}| \ldots{}
%    \begin{macrocode}
\cs_new_protected:Npn \NewExpandableDocumentCommand #1#2#3
  {
    \@@_check_definable:nNT {#1} \NewExpandableDocumentCommand
      {
        \cs_if_exist:NTF #1
          {
            \msg_error:nnee { cmd } { already-defined }
              { \use:nnn \token_to_str:N #1 { } }
              { \token_to_str:N \NewExpandableDocumentCommand }
          }
          { \@@_declare_expandable_cmd:Nnn #1 {#2} {#3} }
      }
  }
\cs_new_protected:Npn \RenewExpandableDocumentCommand #1#2#3
  {
    \@@_check_definable:nNT {#1} \RenewExpandableDocumentCommand
      {
        \cs_if_exist:NTF #1
          { \@@_declare_expandable_cmd:Nnn #1 {#2} {#3} }
          {
            \msg_error:nnee { cmd } { undefined }
              { \use:nnn \token_to_str:N #1 { } }
              { \token_to_str:N \RenewExpandableDocumentCommand }
          }
      }
  }
\cs_new_protected:Npn \ProvideExpandableDocumentCommand #1#2#3
  {
    \@@_check_definable:nNT {#1} \ProvideExpandableDocumentCommand
      {
        \cs_if_exist:NF #1
          { \@@_declare_expandable_cmd:Nnn #1 {#2} {#3} }
      }
 }
\cs_new_protected:Npn \DeclareExpandableDocumentCommand #1#2#3
  {
    \@@_check_definable:nNT {#1} \DeclareExpandableDocumentCommand
      { \@@_declare_expandable_cmd:Nnn #1 {#2} {#3} }
  }
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
%
% \begin{macro}{\IfBooleanT, \IfBooleanF, \IfBooleanTF}
%   The logical \meta{true} and \meta{false} statements are just the
%   normal \cs{c_true_bool} and \cs{c_false_bool} so \cs{bool_if:NTF} is
%   almost enough.  However, this code-level function blows up badly
%   when passed invalid input.  We want \cs{IfBooleanTF} to accept a
%   single (non-space) token equal to \cs{c_true_bool} or
%   \cs{c_false_bool}, possibly surrounded by spaces.  If the input is
%   blank or multiple items, jump to the error and pick the false
%   branch.  If the input, ignoring spaces (we do this by omitting
%   braces in the \cs{tl_if_single_token:nF} test), is not a single
%   token then jump to the error as well.  It is then safe to compare
%   the token to the two booleans, picking the appropriate branch.  If
%   neither matches, we jump to the error as well.
%    \begin{macrocode}
\cs_new:Npn \IfBooleanTF #1
  {
    \tl_if_single:nF {#1}
      { \prg_break:n { \use:n } }
    \tl_if_single_token:nF #1
      { \prg_break:n { \use:n } }
    \token_if_eq_meaning:NNT #1 \c_true_bool
      { \prg_break:n { \use_ii:nnn } }
    \token_if_eq_meaning:NNT #1 \c_false_bool
      { \prg_break:n { \use_iii:nnn } }
    \prg_break:n { \use:n }
    \prg_break_point:
    {
      \msg_expandable_error:nnn { cmd } { if-boolean } {#1}
      \use_ii:nn
    }
  }
\cs_new:Npn \IfBooleanT #1#2 { \IfBooleanTF {#1} {#2} { } }
\cs_new:Npn \IfBooleanF #1 { \IfBooleanTF {#1} { } }
%    \end{macrocode}
% \end{macro}
%
% We adjust here as currently there is still an older version of \cs{NoValue}
% in \pkg{l3tl}.
%
% \begin{macro}{\NoValue}
% \changes{v1.3h}{2025-11-05}{New command}
% \changes{v1.3m}{2026-06-01}{Allow for overwriting \pkg{expl3} version}
%   A special marker; done using \texttt{gset} as there is also a version in
%   \pkg{expl3}, and that has to be in line with what we do here (once
%   everything is synced up, this one will be removed).
%    \begin{macrocode}
\cs_gset_protected:Npn \NoValue { -NoValue- }
%    \end{macrocode}
% \end{macro}
% \begin{variable}{\c_novalue_tl}
% \changes{v1.3h}{2025-11-05}{Update for \cs{NoValue} adjustment}
%    \begin{macrocode}
\cs_undefine:N \c_novalue_tl
\tl_const:Nn \c_novalue_tl { \NoValue }
%    \end{macrocode}
% \end{variable}
% \begin{macro}[pTF, EXP]{\tl_if_novalue:n}
% \changes{v1.3h}{2025-11-05}{Update for \cs{NoValue} adjustment}
% \begin{macro}[EXP]{\@@_if_novalue_aux:w}
% \changes{v1.3h}{2025-11-05}{Update for \cs{NoValue} adjustment}
%   The first argument of \cs{@@_if_novalue_aux:w} is empty if and only if |#1|
%   starts with \cs{NoValue}, while the second argument is empty if |#1| is
%   exactly \cs{NoValue} or if it has a question mark just following the
%   marker. In this second case, however, the material after the  first |?!|
%   remains and makes the emptiness test return \texttt{false}.
%    \begin{macrocode}
%<@@=tl>
\prg_gset_conditional:Npnn \tl_if_novalue:n #1
  { p , T ,  F , TF }
  {
    \@@_if_empty_if:o
      { \@@_if_novalue_aux:w { } #1 { } ? ! \NoValue ? ? ! }
      \prg_return_true:
    \else:
      \prg_return_false:
    \fi:
  }
\cs_gset:Npn \@@_if_novalue_aux:w #1 \NoValue #2 ? #3 ? ! { #1 #2 }
%<@@=cmd>
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}{\IfNoValueT, \IfNoValueF, \IfNoValueTF}
%   Simple re-naming.
%    \begin{macrocode}
\cs_new_eq:NN \IfNoValueF  \tl_if_novalue:nF
\cs_new_eq:NN \IfNoValueT  \tl_if_novalue:nT
\cs_new_eq:NN \IfNoValueTF \tl_if_novalue:nTF
%    \end{macrocode}
% \end{macro}
% \begin{macro}{\IfValueT, \IfValueF, \IfValueTF}
%   Inverted logic.
%    \begin{macrocode}
\cs_new:Npn \IfValueF { \tl_if_novalue:nT }
\cs_new:Npn \IfValueT { \tl_if_novalue:nF }
\cs_new:Npn \IfValueTF #1#2#3 { \tl_if_novalue:nTF {#1} {#3} {#2} }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\IfBlankT, \IfBlankF, \IfBlankTF}
%   Another simple re-naming.
% \changes{v1.0k}{2022/02/19}{Added \cs{IfBlankTF} and friends}
%    \begin{macrocode}
%<latexrelease>\IncludeInRelease{2022/06/01}%
%<latexrelease>                 {\IfBlankTF}{Testing~for~empty~or~blank}%
\cs_new_eq:NN \IfBlankF  \tl_if_blank:nF
\cs_new_eq:NN \IfBlankT  \tl_if_blank:nT
\cs_new_eq:NN \IfBlankTF \tl_if_blank:nTF
%<latexrelease>\EndIncludeInRelease
%<latexrelease>\IncludeInRelease{2021/11/15}%
%<latexrelease>                 {\IfBlankTF}{Testing~for~empty~or~blank}%
%<latexrelease>\cs_undefine:N \IfBlankF
%<latexrelease>\cs_undefine:N \IfBlankT
%<latexrelease>\cs_undefine:N \IfBlankTF
%<latexrelease>
%<latexrelease>\EndIncludeInRelease
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\ProcessedArgument}
%   Processed arguments are returned using this name, which is reserved
%   here although the definition will change.
%    \begin{macrocode}
\tl_new:N \ProcessedArgument
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\ReverseBoolean, \SplitArgument, \SplitList, \TrimSpaces}
%   Simple copies.
%    \begin{macrocode}
\cs_new_eq:NN \ReverseBoolean \@@_bool_reverse:N
\cs_new_eq:NN \SplitArgument  \@@_split_argument:nnn
\cs_new_eq:NN \SplitList      \@@_split_list:nn
\cs_new_eq:NN \TrimSpaces     \@@_trim_spaces:n
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\ProcessList}
%   To support \cs{SplitList}.
% \changes{v1.3b}{2025-02-01}
%                {Use \cs{tl_map_tokens:nn} instead of \cs{tl_map_function:nN}}
%    \begin{macrocode}
\cs_new_eq:NN \ProcessList \tl_map_tokens:nn
%    \end{macrocode}
% \end{macro}
%
% Finally as promised, restore \cs{__kernel_chk_if_free_cs:N}:
%    \begin{macrocode}
%<latexrelease>\cs_gset_eq:NN \__kernel_chk_if_free_cs:N \@@_chk_if_free_cs:N
%<latexrelease>\cs_undefine:N \@@_chk_if_free_cs:N
%    \end{macrocode}
%
%    \begin{macrocode}
%<latexrelease>
%<latexrelease>\IncludeInRelease{0000/00/00}{ltcmd}%
%<latexrelease>                 {Document~command~parser}%
%<latexrelease>
%<latexrelease>\EndModuleRelease
\ExplSyntaxOff
%    \end{macrocode}
%
% Now in |latexrelease| mode, redefine \cs{NewDocumentCommand} to not
% complain on commands already defined.
% \changes{v1.0k}{2022-02-28}
%                {Move latexrelease redefinitions from ltcmd.dtx}
%    \begin{macrocode}
%<latexrelease>\@ifundefined{ExplSyntaxOff}{}{\latexrelease@postltcmd}
%<latexrelease>\catcode`\^^@=\@latexrelease@catcode@null\relax
%</2ekernel|latexrelease>
%    \end{macrocode}
%
% We need to stop DocStrip treating |@@| in a special way at this point.
%    \begin{macrocode}
%<@@=>
%    \end{macrocode}
%
% \Finale

