path: root/Master/texmf-dist/source/latex/l3kernel/l3keys.dtx

% \iffalse meta-comment
%
%% File: l3keys.dtx Copyright (C) 2009-2011 The LaTeX3 Project
%%
%% It may be distributed and/or modified under the conditions of the
%% LaTeX Project Public License (LPPL), either version 1.3c of this
%% license or (at your option) any later version.  The latest version
%% of this license is in the file
%%
%%    http://www.latex-project.org/lppl.txt
%%
%% This file is part of the "expl3 bundle" (The Work in LPPL)
%% and all files in that bundle must be distributed together.
%%
%% The released version of this bundle is available from CTAN.
%%
%% -----------------------------------------------------------------------
%%
%% The development version of the bundle can be found at
%%
%%    http://www.latex-project.org/svnroot/experimental/trunk/
%%
%% for those people who are interested.
%%
%%%%%%%%%%%
%% NOTE: %%
%%%%%%%%%%%
%%
%%   Snapshots taken from the repository represent work in progress and may
%%   not work or may contain conflicting material!  We therefore ask
%%   people _not_ to put them into distributions, archives, etc. without
%%   prior consultation with the LaTeX3 Project.
%%
%% -----------------------------------------------------------------------
%
%<*driver|package>
\RequirePackage{l3names}
\GetIdInfo$Id: l3keys.dtx 2568 2011-08-14 05:03:32Z bruno $
  {L3 Experimental key-value interfaces}
%</driver|package>
%<*driver>
\documentclass[full]{l3doc}
\begin{document}
  \DocInput{\jobname.dtx}
\end{document}
%</driver>
% \fi
%
% \title{^^A
%   The \pkg{l3keys} package\\ Key--value interfaces^^A
%   \thanks{This file describes v\ExplFileVersion,
%      last revised \ExplFileDate.}^^A
% }
%
% \author{^^A
%  The \LaTeX3 Project\thanks
%    {^^A
%      E-mail:
%        \href{mailto:latex-team@latex-project.org}
%          {latex-team@latex-project.org}^^A
%    }^^A
% }
%
% \date{Released \ExplFileDate}
%
% \maketitle
%
% \begin{documentation}
%
% The key--value method is a popular system for creating large numbers
% of settings for controlling function or package behaviour.  For the
% user, the system normally results in input of the form
% \begin{verbatim}
%   \PackageControlMacro{
%     key-one = value one,
%     key-two = value two
%   }
% \end{verbatim}
% or
% \begin{verbatim}
%   \PackageMacro[
%     key-one = value one,
%     key-two = value two
%   ]{argument}.
% \end{verbatim}
%
% The high level functions here are intended as a method to create
% key--value controls. Keys are themselves created using a key--value
% interface, minimising the number of functions and arguments
% required. Each key is created by setting one or more \emph{properties}
% of the key:
% \begin{verbatim}
%   \keys_define:nn { module }
%     {
%       key-one .code:n   = code including parameter #1,
%       key-two .tl_set:N = \l_module_store_tl
%     }
% \end{verbatim}
% These values can then be set as with other key--value approaches:
% \begin{verbatim}
%   \keys_set:nn { module }
%     {
%       key-one = value one,
%       key-two = value two
%     }
% \end{verbatim}
%
% At a document level, \cs{keys_set:nn} will be used within a
% document function, for example
% \begin{verbatim}
%   \DeclareDocumentCommand \SomePackageSetup { m }
%     { \keys_set:nn { module } { #1 }  }
%  \DeclareDocumentCommand \SomePackageMacro { o m }
%    {
%      \group_begin:
%        \keys_set:nn { module } { #1 }
%        % Main code for \SomePackageMacro
%      \group_end:
%    }
% \end{verbatim}
%
% Key names may contain any tokens, as they are handled internally
% using \cs{tl_to_str:n}. As will be discussed in
% section~\ref{sec:subdivision}, it is suggested that the character
% |/| is reserved for sub-division of keys into logical
% groups. Functions and variables are \emph{not} expanded when creating
% key names, and so
% \begin{verbatim}
%   \tl_set:Nn \l_module_tmp_tl { key }
%   \keys_define:nn { module }
%     {
%       \l_module_tmp_tl .code:n = code
%     }
% \end{verbatim}
% will create a key called \cs{l_module_tmp_tl}, and not one called
% \texttt{key}.
%
% \section{Creating keys}
%
% \begin{function}{\keys_define:nn}
%   \begin{syntax}
%     \cs{keys_define:nn} \Arg{module} \Arg{keyval list}
%   \end{syntax}
%   Parses the \meta{keyval list} and defines the keys listed there for
%   \meta{module}. The \meta{module} name should be a text value, but
%   there are no restrictions on the nature of the text. In practice the
%   \meta{module} should be chosen to be unique to the module in question
%   (unless deliberately adding keys to an existing module).
%
%   The \meta{keyval list} should consist of one or more key names along
%   with an associated key \emph{property}. The properties of a key
%   determine how it acts. The individual properties are described
%   in the following text; a typical use of \cs{keys_define:nn} might
%   read
%   \begin{verbatim}
%     \keys_define:nn { mymodule }
%       {
%         keyname .code:n = Some~code~using~#1,
%         keyname .value_required:
%       }
%   \end{verbatim}
%   where the properties of the key begin from the |.| after the key
%   name.
% \end{function}
%
% The various properties available take either no arguments at
% all, or require exactly one argument. This is indicated in the
% name of the property using an argument specification. In the following
% discussion, each property is illustrated attached to an
% arbitrary \meta{key}, which when used may be supplied with a
% \meta{value}. All key \emph{definitions} are local.
%
% \begin{function}{.bool_set:N}
%   \begin{syntax}
%     \meta{key} .bool_set:N = \meta{boolean}
%   \end{syntax}
%   Defines \meta{key} to set \meta{boolean} to \meta{value} (which
%   must be either \texttt{true} or \texttt{false}).  If the variable
%   does not exist, it will be created at the point that the key is
%   set up. The \meta{boolean} will be assigned locally.
% \end{function}
%
% \begin{function}{.bool_gset:N}
%   \begin{syntax}
%     \meta{key} .bool_gset:N = \meta{boolean}
%   \end{syntax}
%   Defines \meta{key} to set \meta{boolean} to \meta{value} (which
%   must be either \texttt{true} or \texttt{false}).  If the variable
%   does not exist, it will be created at the point that the key is
%   set up. The \meta{boolean} will be assigned globally.
% \end{function}
%
% \begin{function}{.choice:}
%   \begin{syntax}
%     \meta{key} .choice:
%   \end{syntax}
%   Sets \meta{key} to act as a multiple choice key. Each valid choice
%   for \meta{key} must then be created, as discussed in
%   section~\ref{sec:choice}.
% \end{function}
%
% \begin{function}{.choice_code:n, .choice_code:x}
%   \begin{syntax}
%     \meta{key} .choice_code:n = \meta{code}
%   \end{syntax}
%   Stores \meta{code} for use when \texttt{.generate_choices:n} creates
%   one or more choice sub-keys of the current key. Inside \meta{code},
%   \cs{l_keys_choice_tl} will expand to the name of the choice made, and
%   \cs{l_keys_choice_int} will be the position of the choice in the list
%   given to \texttt{.generate_choices:n}. Choices are discussed in
%   detail in section~\ref{sec:choice}.
% \end{function}
%
% \begin{function}{.code:n, .code:x}
%   \begin{syntax}
%     \meta{key} .code:n = \meta{code}
%   \end{syntax}
%   Stores the \meta{code} for execution when \meta{key} is used. The
%   The \meta{code} can include one parameter ("#1"), which will be the
%   \meta{value} given for the \meta{key}. The \texttt{x}-type variant
%   will expand \meta{code} at the point  where the \meta{key} is
%   created.
% \end{function}
%
% \begin{function}{.default:n, .default:V}
%   \begin{syntax}
%     \meta{key} .default:n = \meta{default}
%   \end{syntax}
%   Creates a \meta{default} value for \meta{key}, which is used if no
%   value is given. This will be used if only the key name is given,
%   but not if a blank \meta{value} is given:
%   \begin{verbatim}
%     \keys_define:nn { module }
%       {
%         key .code:n    = Hello~#1,
%         key .default:n = World
%       }
%     \keys_set:nn { module }
%       {
%         key = Fred, % Prints 'Hello Fred'
%         key,        % Prints 'Hello World'
%         key = ,     % Prints 'Hello '
%       }
%   \end{verbatim}
% \end{function}
%
% \begin{function}{.dim_set:N, .dim_set:c}
%   \begin{syntax}
%     \meta{key} .dim_set:N = \meta{dimension}
%   \end{syntax}
%   Defines \meta{key} to set \meta{dimension} to \meta{value} (which
%   must a dimension expression).  If the variable does not exist, it
%   will be created at the point that the key is set up. The
%   \meta{dimension} will be assigned locally.
% \end{function}
%
% \begin{function}{.dim_gset:N, .dim_gset:c}
%   \begin{syntax}
%     \meta{key} .dim_gset:N = \meta{dimension}
%   \end{syntax}
%   Defines \meta{key} to set \meta{dimension} to \meta{value} (which
%   must a dimension expression).  If the variable does not exist, it
%   will be created at the point that the key is set up. The
%   \meta{dimension} will be assigned globally.
% \end{function}
%
% \begin{function}{.fp_set:N, .fp_set:c}
%   \begin{syntax}
%     \meta{key} .fp_set:N = \meta{floating point}
%   \end{syntax}
%   Defines \meta{key} to set \meta{floating point} to \meta{value}
%   (which must a floating point number).  If the variable does not exist,
%   it will be created at the point that the key is set up. The
%   \meta{integer} will be assigned locally.
% \end{function}
%
% \begin{function}{.fp_gset:N, .fp_gset:c}
%   \begin{syntax}
%     \meta{key} .fp_gset:N = \meta{floating point}
%   \end{syntax}
%   Defines \meta{key} to set \meta{floating-point} to \meta{value}
%   (which must a floating point number).  If the variable does not exist,
%   it will be created at the point that the key is set up. The
%   \meta{integer} will be assigned globally.
% \end{function}
%
% \begin{function}{.generate_choices:n}
%   \begin{syntax}
%     \meta{key} .generate_choices:n = \Arg{list}
%   \end{syntax}
%   This property will mark \meta{key} as a multiple choice key,
%   and will use the \meta{list} to define the choices. The \meta{list}
%   should consist of a comma-separated list of choice names. Each
%   choice will be set up to execute \meta{code} as set using
%   \texttt{.choice_code:n} (or \texttt{.choice_code:x}). Choices are
%   discussed in detail in section~\ref{sec:choice}.
% \end{function}
%
% \begin{function}{.int_set:N, .int_set:c}
%   \begin{syntax}
%     \meta{key} .int_set:N = \meta{integer}
%   \end{syntax}
%   Defines \meta{key} to set \meta{integer} to \meta{value} (which
%   must be an integer expression).  If the variable does not exist, it
%   will be created at the point that the key is set up. The
%   \meta{integer} will be assigned locally.
% \end{function}
%
% \begin{function}{.int_gset:N, .int_gset:c}
%   \begin{syntax}
%     \meta{key} .int_gset:N = \meta{integer}
%   \end{syntax}
%   Defines \meta{key} to set \meta{integer} to \meta{value} (which
%   must be an integer expression).  If the variable does not exist, it
%   will be created at the point that the key is set up. The
%   \meta{integer} will be assigned globally.
% \end{function}
%
% \begin{function}{.meta:n, .meta:x}
%   \begin{syntax}
%     \meta{key} .meta:n = \Arg{keyval list}
%   \end{syntax}
%   Makes \meta{key} a meta-key, which will set \meta{keyval list} in
%   one go.  If \meta{key} is given with a value at the time the key
%   is used, then the value will be passed through to the subsidiary
%   \meta{keys} for processing (as |#1|).
% \end{function}
%
% \begin{function}{.skip_set:N, .skip_set:c}
%   \begin{syntax}
%     \meta{key} .skip_set:N = \meta{skip}
%   \end{syntax}
%   Defines \meta{key} to set \meta{skip} to \meta{value} (which
%   must be a skip expression). If the variable does not exist, it
%   will be created at the point that the key is set up. The
%   \meta{skip} will be assigned locally.
% \end{function}
%
% \begin{function}{.skip_gset:N, .skip_gset:c}
%   \begin{syntax}
%     \meta{key} .skip_gset:N = \meta{skip}
%   \end{syntax}
%   Defines \meta{key} to set \meta{skip} to \meta{value} (which
%   must be a skip expression). If the variable does not exist, it
%   will be created at the point that the key is set up. The
%   \meta{skip} will be assigned globally.
% \end{function}
%
% \begin{function}{.tl_set:N, .tl_set:c}
%   \begin{syntax}
%     \meta{key} .tl_set:N = \meta{token list variable}
%   \end{syntax}
%   Defines \meta{key} to set \meta{token list variable} to \meta{value}.
%   If the variable does not exist, it will be created at the point that
%   the key is set up. The \meta{token list variable} will be assigned
%   locally.
% \end{function}
%
% \begin{function}{.tl_gset:N, .tl_gset:c}
%   \begin{syntax}
%     \meta{key} .tl_gset:N = \meta{token list variable}
%   \end{syntax}
%   Defines \meta{key} to set \meta{token list variable} to \meta{value}.
%   If the variable does not exist, it will be created at the point that
%   the key is set up. The \meta{token list variable} will be assigned
%   globally.
% \end{function}
%
% \begin{function}{.tl_set_x:N, .tl_set_x:c}
%   \begin{syntax}
%     \meta{key} .tl_set_x:N = \meta{token list variable}
%   \end{syntax}
%   Defines \meta{key} to set \meta{token list variable} to \meta{value},
%   which will be subjected to an \texttt{x}-type expansion
%   (\emph{i.e.}~using \cs{tl_set:Nx}). If the variable does not exist,
%   it will be created at the point that the key is set up. The
%   \meta{token list variable} will be assigned locally.
% \end{function}
%
% \begin{function}{.tl_gset_x:N, .tl_gset_x:c}
%   \begin{syntax}
%     \meta{key} .tl_gset_x:N = \meta{token list variable}
%   \end{syntax}
%   Defines \meta{key} to set \meta{token list variable} to \meta{value},
%   which will be subjected to an \texttt{x}-type expansion
%   (\emph{i.e.}~using \cs{tl_set:Nx}). If the variable does not exist,
%   it will be created at the point that the key is set up. The
%   \meta{token list variable} will be assigned globally.
% \end{function}
%
% \begin{function}{.value_forbidden:}
%   \begin{syntax}
%     \meta{key} .value_forbidden:
%   \end{syntax}
%   Specifies that \meta{key} cannot receive a \meta{value} when used.
%   If a \meta{value} is given then an error will be issued.
% \end{function}
%
% \begin{function}{.value_required:}
%   \begin{syntax}
%      \meta{key} .value_required:
%   \end{syntax}
%   Specifies that \meta{key} must receive a \meta{value} when used.
%   If a \meta{value} is not given then an error will be issued.
% \end{function}
%
% \section{Sub-dividing keys}
% \label{sec:subdivision}
%
% When creating large numbers of keys, it may be desirable to divide
% them into several sub-groups for a given module. This can be achieved
% either by adding a sub-division to the module name:
% \begin{verbatim}
%   \keys_define:nn { module / subgroup }
%     { key .code:n = code }
% \end{verbatim}
% or to the key name:
% \begin{verbatim}
%   \keys_define:nn { module }
%     { subgroup / key .code:n = code }
% \end{verbatim}
% As illustrated, the best choice of token for sub-dividing keys in
% this way is |/|. This is because of the method that is
% used to represent keys internally. Both of the above code fragments
% set the same key, which has full name \texttt{module/subgroup/key}.
%
% As will be illustrated in the next section, this subdivision is
% particularly relevant to making multiple choices.
%
% \section{Multiple choice keys}
% \label{sec:choice}
%
% Multiple choices are created by setting the \texttt{.choice:}
% property:
% \begin{verbatim}
%   \keys_define:nn { module }
%     { key .choice: }
% \end{verbatim}
% For keys which are set up as choices, the valid choices are generated
% by creating sub-keys of the choice key. This can be carried out in
% two ways.
%
% In many cases, choices execute similar code which is dependant only
% on the name of the choice or the position of the choice in the
% list of choices. Here, the keys can share the same code, and can
% be rapidly created using the  \texttt{.choice_code:n} and
% \texttt{.generate_choices:n} properties:
% \begin{verbatim}
%   \keys_define:nn { module }
%     {
%       key .choice_code:n =
%         {
%           You~gave~choice~'\int_use:N \l_keys_choice_tl',~
%           which~is~in~position~
%           \int_use:N \l_keys_choice_int \c_space_tl
%           in~the~list.
%         },
%       key .generate_choices:n =
%         { choice-a, choice-b, choice-c }
%     }
% \end{verbatim}
% Following common computing practice, \cs{l_keys_choice_int} is
% indexed from  $0$ (as an offset), so that the value of
% \cs{l_keys_choice_int} for the first choice in a list will be
% zero.
%
% \begin{variable}{\l_keys_choice_int, \l_keys_choice_tl}
%   Inside the code block for a choice generated using
%   \texttt{.generate_choice:}, the variables \cs{l_keys_choice_tl} and
%   \cs{l_keys_choice_int} are available to indicate the name of the
%   current choice, and its position in the comma list.  The position
%   is indexed from $0$.
% \end{variable}
%
% On the other hand, it is sometimes useful to create choices which
% use entirely different code from one another. This can be achieved
% by setting the \texttt{.choice:} property of a key, then manually
% defining sub-keys.
% \begin{verbatim}
%   \keys_define:nn { module }
%     {
%       key .choice:n,
%       key / choice-a .code:n = code-a,
%       key / choice-b .code:n = code-b,
%       key / choice-c .code:n = code-c,
%     }
%\end{verbatim}
%
% It is possible to mix the two methods, but manually-created choices
% should \emph{not} use \cs{l_keys_choice_tl} or \cs{l_keys_choice_int}.
% These variables do not have defined behaviour when used outside of
% code created using \texttt{.generate_choices:n}
% (\emph{i.e.}~anything might happen).
%
% \section{Setting keys}
%
% \begin{function}{\keys_set:nn, \keys_set:nV, \keys_set:nv, \keys_set:no}
%   \begin{syntax}
%     \cs{keys_set:nn} \Arg{module} \Arg{keyval list}
%   \end{syntax}
%   Parses the \meta{keyval list}, and sets those keys which are defined
%   for \meta{module}. The behaviour on finding an unknown key can be set
%   by defining a special \texttt{unknown} key: this will be illustrated
%   later. In contrast to \cs{keys_define:nn}, this function does check
%   category codes and ignore spaces, and is therefore suitable for user
%   input.
% \end{function}
%
% If a key is not known, \cs{keys_set:nn} will look for a special
% \texttt{unknown} key for the same module. This mechanism can be
% used to create new keys from user input.
% \begin{verbatim}
%   \keys_define:nn { module }
%     {
%       unknown .code:n =
%         You~tried~to~set~key~'\l_keys_key_tl'~to~'#1'.
%     }
%\end{verbatim}
%
% \begin{variable}{\l_keys_key_tl}
%   When processing an unknown key, the name of the key is available
%   as \cs{l_keys_key_tl}. Note that this will have been processed
%   using \cs{tl_to_str:n}. The value passed to the key (if any) is
%   available as the macro parameter |#1|.
% \end{variable}
%
% \begin{variable}{\l_keys_path_tl}
%   When processing an unknown key, the path of the key used is available
%   as \cs{l_keys_path_tl}. Note that this will have been processed
%   using \cs{tl_to_str:n}.
% \end{variable}
%
% \section{Utility functions for keys}
%
% \begin{function}[EXP,pTF]{\keys_if_exist:nn}
%   \begin{syntax}
%     \cs{keys_if_exist:nn} \meta{module} \meta{key}
%     \cs{keys_if_exist:nn} \meta{module} \meta{key}
%     ~~\Arg{true code} \Arg{false code}
%   \end{syntax}
%   Tests if the \meta{key} exists for \meta{module}, \emph{i.e.}~if any code
%   has been defined for \meta{key}.
%   The branching versions then leave either \meta{true code} or
%   \meta{false code} in the input stream, as appropriate to the truth
%   of the test and the variant of the function chosen. The logical
%   truth of the test is left in the input stream by the predicate
%   version.
% \end{function}
%
% \begin{function}{\keys_show:nn}
%   \begin{syntax}
%     \cs{keys_show:nn} \Arg{module} \Arg{key}
%   \end{syntax}
%   Shows the function which is used to actually implement a
%   \meta{key} for a \meta{module}.
% \end{function}
%
% \end{documentation}
%
% \begin{implementation}
%
% \section{\pkg{l3keys} Implementation}
%
%    \begin{macrocode}
%<*initex|package>
%    \end{macrocode}
%
%    \begin{macrocode}
%<*package>
\ProvidesExplPackage
  {\ExplFileName}{\ExplFileDate}{\ExplFileVersion}{\ExplFileDescription}
\package_check_loaded_expl:
%</package>
%    \end{macrocode}
%
% \subsection{Constants and variables}
%
% \begin{variable}{\c_keys_code_root_tl, \c_keys_vars_root_tl}
%   The prefixes for the code and variables of the keys themselves.
%    \begin{macrocode}
\tl_const:Nn \c_keys_code_root_tl { key~code~>~ }
\tl_const:Nn \c_keys_vars_root_tl { key~var~>~ }
%    \end{macrocode}
% \end{variable}
%
% \begin{variable}{\c_keys_props_root_tl}
%   The prefix for storing properties.
%    \begin{macrocode}
\tl_const:Nn \c_keys_props_root_tl { key~prop~>~ }
%    \end{macrocode}
% \end{variable}
%
% \begin{variable}{\c_keys_value_forbidden_tl, \c_keys_value_required_tl}
%   Two marker token lists.
%    \begin{macrocode}
\tl_const:Nn \c_keys_value_forbidden_tl { forbidden }
\tl_const:Nn \c_keys_value_required_tl  { required }
%    \end{macrocode}
% \end{variable}
%
% \begin{variable}{\l_keys_choice_int, \l_keys_choices_tl}
%   Publicly accessible data on which choice is being used when several
%   are generated as a set.
%    \begin{macrocode}
\int_new:N \l_keys_choice_int
\tl_new:N \l_keys_choices_tl
%    \end{macrocode}
% \end{variable}
%
% \begin{variable}{\l_keys_key_tl}
%   The name of a key itself: needed when setting keys.
%    \begin{macrocode}
\tl_new:N \l_keys_key_tl
%    \end{macrocode}
% \end{variable}
%
% \begin{variable}{\l_keys_module_tl}
%   The module for an entire set of keys.
%    \begin{macrocode}
\tl_new:N \l_keys_module_tl
%    \end{macrocode}
% \end{variable}
%
% \begin{variable}{\l_keys_no_value_bool}
%   A marker is needed internally to show if only a key or a key plus a
%   value was seen: this is recorded here.
%    \begin{macrocode}
\bool_new:N \l_keys_no_value_bool
%    \end{macrocode}
% \end{variable}
%
% \begin{variable}{\l_keys_path_tl}
%   The \enquote{path} of the current key is stored here: this is
%   available to the programmer and so is public.
%    \begin{macrocode}
\tl_new:N \l_keys_path_tl
%    \end{macrocode}
% \end{variable}
%
% \begin{variable}{\l_keys_property_tl}
%   The \enquote{property} begin set for a key at definition time is
%   stored here.
%    \begin{macrocode}
\tl_new:N \l_keys_property_tl
%    \end{macrocode}
% \end{variable}
%
% \begin{variable}{\l_keys_value_tl}
%   The value given for a key: may be empty if no value was given.
%    \begin{macrocode}
\tl_new:N \l_keys_value_tl
%    \end{macrocode}
% \end{variable}
%
% \subsection{The key defining mechanism}
%
% \begin{macro}{\keys_define:nn}
% \begin{macro}[aux]{\keys_define_aux:nnn, \keys_define_aux:onn}
%   The public function for definitions is just a wrapper for the lower
%   level mechanism, more or less. The outer function is designed to
%   keep a track of the current module, to allow safe nesting.
%    \begin{macrocode}
\cs_new_protected:Npn \keys_define:nn
  { \keys_define_aux:onn \l_keys_module_tl }
\cs_new_protected:Npn \keys_define_aux:nnn #1#2#3
  {
    \tl_set:Nx \l_keys_module_tl { \tl_to_str:n {#2} }
    \keyval_parse:NNn \keys_define_elt:n \keys_define_elt:nn {#3}
    \tl_set:Nn \l_keys_module_tl {#1}
  }
\cs_generate_variant:Nn \keys_define_aux:nnn { o }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}[int]{\keys_define_elt:n}
% \begin{macro}[int]{\keys_define_elt:nn}
% \begin{macro}[aux]{\keys_define_elt_aux:nn}
%   The outer functions here record whether a value was given and then
%   converge on a common internal mechanism. There is first a search for
%   a property in the current key name, then a check to make sure it is
%   known before the code hands off to the next step.
%    \begin{macrocode}
\cs_new_protected_nopar:Npn \keys_define_elt:n #1
  {
    \bool_set_true:N \l_keys_no_value_bool
    \keys_define_elt_aux:nn {#1} { }
  }
\cs_new_protected:Npn \keys_define_elt:nn #1#2
  {
    \bool_set_false:N \l_keys_no_value_bool
    \keys_define_elt_aux:nn {#1} {#2}
  }
\cs_new_protected:Npn \keys_define_elt_aux:nn #1#2 {
  \keys_property_find:n {#1}
  \cs_if_exist:cTF { \c_keys_props_root_tl \l_keys_property_tl }
    { \keys_define_key:n {#2} }
    {
      \msg_kernel_error:nnxx { keys } { property-unknown }
        { \l_keys_property_tl } { \l_keys_path_tl }
    }
}
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \end{macro}
%
% \begin{macro}[int]{\keys_property_find:n}
% \begin{macro}[aux]{\keys_property_find_aux:w}
%   Searching for a property means finding the last |.| in the input,
%   and storing the text before and after it. Everything is turned into
%   strings, so there is no problem using an \texttt{x}-type expansion.
%    \begin{macrocode}
\cs_new_protected_nopar:Npn \keys_property_find:n #1
  {
    \tl_set:Nx \l_keys_path_tl { \l_keys_module_tl / }
    \tl_if_in:nnTF {#1} { . }
      { \keys_property_find_aux:w #1 \q_stop }
      { \msg_kernel_error:nnx { keys } { key-no-property } {#1} }
  }
\cs_new_protected_nopar:Npn \keys_property_find_aux:w #1 . #2 \q_stop
  {
    \tl_set:Nx \l_keys_path_tl { \l_keys_path_tl \tl_to_str:n {#1} }
    \tl_if_in:nnTF {#2} { . }
      {
        \tl_set:Nx \l_keys_path_tl { \l_keys_path_tl . }
        \keys_property_find_aux:w #2 \q_stop
      }
      { \tl_set:Nn \l_keys_property_tl { . #2 } }
  }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}[int]{\keys_define_key:n}
% \begin{macro}[aux]{\keys_define_key_aux:w}
%   Two possible cases. If there is a value for the key, then just use
%   the function. If not, then a check to make sure there is no need for
%   a value with the property. If there should be one then complain,
%   otherwise execute it. There is no need to check for a |:| as if it
%   is missing the earlier tests will have failed.
%    \begin{macrocode}
\cs_new_protected:Npn \keys_define_key:n #1
  {
    \bool_if:NTF \l_keys_no_value_bool
      {
        \exp_after:wN \keys_define_key_aux:w
          \l_keys_property_tl \q_stop
          { \use:c { \c_keys_props_root_tl \l_keys_property_tl } }
          {
            \msg_kernel_error:nnxx { keys }
              { property-requires-value } { \l_keys_property_tl }
              { \l_keys_path_tl }
            }
      }
      { \use:c { \c_keys_props_root_tl \l_keys_property_tl } {#1} }
  }
\cs_new_protected:Npn \keys_define_key_aux:w #1 : #2 \q_stop
  { \tl_if_empty:nTF {#2} }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \subsection{Turning properties into actions}
%
% \begin{macro}[int]{\keys_bool_set:NN}
%   Boolean keys are really just choices, but all done by hand. The
%   second argument here is the scope: either empty or \texttt{g} for
%   global.
%    \begin{macrocode}
\cs_new_nopar:Npn \keys_bool_set:NN #1#2
  {
    \cs_if_exist:NF #1 { \bool_new:N #1 }
    \keys_choice_make:
    \keys_cmd_set:nx { \l_keys_path_tl / true }
      { \exp_not:c { bool_ #2 set_true:N } \exp_not:N #1 }
    \keys_cmd_set:nx { \l_keys_path_tl / false }
      { \exp_not:c { bool_ #2 set_false:N } \exp_not:N #1 }
    \keys_cmd_set:nn { \l_keys_path_tl / unknown }
      {
        \msg_kernel_error:nnx { keys } { boolean-values-only }
          { \l_keys_key_tl }
      }
    \keys_default_set:n { true }
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}[int]{\keys_choice_make:}
%   To make a choice from a key, two steps: set the code, and set the
%   unknown key.
%    \begin{macrocode}
\cs_new_protected_nopar:Npn \keys_choice_make:
  {
    \keys_cmd_set:nn { \l_keys_path_tl }
      { \keys_choice_find:n {##1} }
    \keys_cmd_set:nn { \l_keys_path_tl / unknown }
      {
        \msg_kernel_error:nnxx { keys } { choice-unknown }
          { \l_keys_path_tl } {##1}
      }
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}[int]{\keys_choices_generate:n}
% \begin{macro}[aux]{\keys_choices_generate_aux:n}
%   Creating multiple-choices means setting up the \enquote{indicator}
%   code, then applying whatever the user wanted.
%    \begin{macrocode}
\cs_new_protected:Npn \keys_choices_generate:n #1
  {
    \cs_if_exist:cTF
      { \c_keys_vars_root_tl \l_keys_path_tl .choice~code }
      {
        \keys_choice_make:
        \int_zero:N \l_keys_choice_int
        \clist_map_function:nN {#1} \keys_choices_generate_aux:n
      }
      {
        \msg_kernel_error:nnx { keys }
          { generate-choices-before-code } { \l_keys_path_tl }
      }
  }
\cs_new_protected_nopar:Npn \keys_choices_generate_aux:n #1
  {
    \keys_cmd_set:nx { \l_keys_path_tl / #1 }
      {
        \exp_not:n { \tl_set:Nn \l_keys_choice_tl } {#1}
        \exp_not:n { \int_set:Nn \l_keys_choice_int }
          { \int_use:N \l_keys_choice_int }
        \exp_not:v
          { \c_keys_vars_root_tl \l_keys_path_tl .choice~code }
      }
    \int_incr:N \l_keys_choice_int
  }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}[int]{\keys_choice_code_store:x}
%   The code for making multiple choices is stored in a token list.
%    \begin{macrocode}
\cs_new_protected:Npn \keys_choice_code_store:x #1
  {
    \cs_if_exist:cF
      { \c_keys_vars_root_tl \l_keys_path_tl .choice~code }
      {
        \tl_new:c
          { \c_keys_vars_root_tl \l_keys_path_tl .choice~code }
      }
    \tl_set:cx { \c_keys_vars_root_tl \l_keys_path_tl .choice~code }
      {#1}
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}[int]{\keys_cmd_set:nn, \keys_cmd_set:nx}
% \begin{macro}[aux]{\keys_cmd_set_aux:n}
%   Creating a new command means tidying up the properties and then making
%   the internal function which actually does the work.
%    \begin{macrocode}
\cs_new_protected:Npn \keys_cmd_set:nn #1#2
  {
    \keys_cmd_set_aux:n {#1}
    \cs_set:cpn { \c_keys_code_root_tl #1 } ##1 {#2}
  }
\cs_new_protected:Npn \keys_cmd_set:nx #1#2
  {
    \keys_cmd_set_aux:n {#1}
    \cs_set:cpx { \c_keys_code_root_tl #1 } ##1 {#2}
  }
\cs_new_protected_nopar:Npn \keys_cmd_set_aux:n #1
  {
    \tl_clear_new:c { \c_keys_vars_root_tl #1 .default }
    \tl_set:cn { \c_keys_vars_root_tl #1 .default } { \q_no_value }
    \tl_clear_new:c { \c_keys_vars_root_tl #1 .req }
  }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}[int]{\keys_default_set:n, \keys_default_set:V}
%   Setting a default value is easy.
%    \begin{macrocode}
\cs_new_protected:Npn \keys_default_set:n #1
  { \tl_set:cn { \c_keys_vars_root_tl \l_keys_path_tl .default } {#1} }
\cs_generate_variant:Nn \keys_default_set:n { V }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\keys_meta_make:n, \keys_meta_make:x}
%   To create a meta-key, simply set up to pass data through.
%    \begin{macrocode}
\cs_new_protected_nopar:Npn \keys_meta_make:n #1
  {
    \exp_args:NNo \keys_cmd_set:nn \l_keys_path_tl
      { \exp_after:wN \keys_set:nn  \exp_after:wN { \l_keys_module_tl } {#1} }
  }
\cs_new_protected_nopar:Npn \keys_meta_make:x #1
  {
    \keys_cmd_set:nx { \l_keys_path_tl }
      { \exp_not:N \keys_set:nn { \l_keys_module_tl } {#1} }
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}[int]{\keys_value_requirement:n}
%   Values can be required or forbidden by having the appropriate marker
%   set.
%    \begin{macrocode}
\cs_new_protected_nopar:Npn \keys_value_requirement:n #1
  {
    \tl_set_eq:cc
      { \c_keys_vars_root_tl \l_keys_path_tl .req }
      { c_keys_value_ #1 _tl }
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}[int]{\keys_variable_set:NnNN, \keys_variable_set:cnNN}
% \begin{macro}[int]{\keys_variable_set:NnN, \keys_variable_set:cnN}
%   Setting a variable takes the type and scope separately so that
%   it is easy to make a new variable if needed. The three-argument
%   version is set up so that the use of |{ }| as an \texttt{N}-type
%   variable is only done once!
%    \begin{macrocode}
\cs_new_protected_nopar:Npn \keys_variable_set:NnNN #1#2#3#4
  {
    \cs_if_exist:NF #1 { \use:c { #2 _new:N } #1 }
    \keys_cmd_set:nx { \l_keys_path_tl }
      { \exp_not:c { #2 _ #3 set:N #4 } \exp_not:N #1 {##1} }
  }
\cs_new_protected_nopar:Npn \keys_variable_set:NnN #1#2#3
  { \keys_variable_set:NnNN #1 {#2} { } #3 }
\cs_generate_variant:Nn \keys_variable_set:NnNN { c }
\cs_generate_variant:Nn \keys_variable_set:NnN  { c }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \subsection{Creating key properties}
%
% The key property functions are all wrappers for internal functions,
% meaning that things stay readable and can also be altered later on.
%
% \begin{macro}{.bool_set:N}
% \begin{macro}{.bool_gset:N}
%   One function for this.
%    \begin{macrocode}
\cs_new_protected_nopar:cpn { \c_keys_props_root_tl .bool_set:N } #1
  { \keys_bool_set:NN #1 { } }
\cs_new_protected_nopar:cpn { \c_keys_props_root_tl .bool_gset:N } #1
  { \keys_bool_set:NN #1 g }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}{.choice:}
%   Making a choice is handled internally, as it is also needed by
% \texttt{.generate_choices:n}.
%    \begin{macrocode}
\cs_new_protected_nopar:cpn { \c_keys_props_root_tl .choice: }
  { \keys_choice_make: }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{.code:n, .code:x}
%   Creating code is simply a case of passing through to the underlying
%   \texttt{set} function.
%    \begin{macrocode}
\cs_new_protected:cpn { \c_keys_props_root_tl .code:n } #1
  { \keys_cmd_set:nn { \l_keys_path_tl } {#1} }
\cs_new_protected:cpn { \c_keys_props_root_tl .code:x } #1
  { \keys_cmd_set:nx { \l_keys_path_tl } {#1} }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{.choice_code:n, .choice_code:x}
%   Storing the code for choices, using \cs{exp_not:n} to avoid needing
%   two internal functions.
%    \begin{macrocode}
\cs_new_protected:cpn { \c_keys_props_root_tl .choice_code:n } #1
  { \keys_choice_code_store:x { \exp_not:n {#1} } }
\cs_new_protected:cpn { \c_keys_props_root_tl .choice_code:x } #1
  { \keys_choice_code_store:x {#1} }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{.default:n, .default:V}
%   Expansion is left to the internal functions.
%    \begin{macrocode}
\cs_new_protected:cpn { \c_keys_props_root_tl .default:n } #1
  { \keys_default_set:n {#1} }
\cs_new_protected:cpn { \c_keys_props_root_tl .default:V } #1
  { \keys_default_set:V #1 }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{.dim_set:N, .dim_set:c}
% \begin{macro}{.dim_gset:N, .dim_gset:c}
% Setting a variable is very easy: just pass the data along.
%    \begin{macrocode}
\cs_new_protected_nopar:cpn { \c_keys_props_root_tl .dim_set:N } #1
  { \keys_variable_set:NnN #1 { dim } n }
\cs_new_protected_nopar:cpn { \c_keys_props_root_tl .dim_set:c } #1
  { \keys_variable_set:cnN {#1} { dim } n }
\cs_new_protected_nopar:cpn { \c_keys_props_root_tl .dim_gset:N } #1
  { \keys_variable_set:NnNN #1 { dim } g n }
\cs_new_protected_nopar:cpn { \c_keys_props_root_tl .dim_gset:c } #1
  { \keys_variable_set:cnNN {#1} { dim } g n }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}{.fp_set:N, .fp_set:c}
% \begin{macro}{.fp_gset:N, .fp_gset:c}
%   Setting a variable is very easy: just pass the data along.
%    \begin{macrocode}
\cs_new_protected_nopar:cpn { \c_keys_props_root_tl .fp_set:N } #1
  { \keys_variable_set:NnN #1 { fp } n }
\cs_new_protected_nopar:cpn { \c_keys_props_root_tl .fp_set:c } #1
  { \keys_variable_set:cnN {#1} { fp } n }
\cs_new_protected_nopar:cpn { \c_keys_props_root_tl .fp_gset:N } #1
  { \keys_variable_set:NnNN #1 { fp } g n }
\cs_new_protected_nopar:cpn { \c_keys_props_root_tl .fp_gset:c } #1
  { \keys_variable_set:cnNN {#1} { fp } g n }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}{.generate_choices:n}
%   Making choices is easy.
%    \begin{macrocode}
\cs_new_protected:cpn { \c_keys_props_root_tl .generate_choices:n } #1
  { \keys_choices_generate:n {#1} }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{.int_set:N, .int_set:c}
% \begin{macro}{.int_gset:N, .int_gset:c}
%   Setting a variable is very easy: just pass the data along.
%    \begin{macrocode}
\cs_new_protected_nopar:cpn { \c_keys_props_root_tl .int_set:N } #1
  { \keys_variable_set:NnN #1 { int } n }
\cs_new_protected_nopar:cpn { \c_keys_props_root_tl .int_set:c } #1
  { \keys_variable_set:cnN {#1} { int } n }
\cs_new_protected_nopar:cpn { \c_keys_props_root_tl .int_gset:N } #1
  { \keys_variable_set:NnNN #1 { int } g n }
\cs_new_protected_nopar:cpn { \c_keys_props_root_tl .int_gset:c } #1
  { \keys_variable_set:cnNN {#1} { int } g n }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
%\begin{macro}{.meta:n, .meta:x}
% Making a meta is handled internally.
%    \begin{macrocode}
\cs_new_protected:cpn { \c_keys_props_root_tl .meta:n } #1
  { \keys_meta_make:n {#1} }
\cs_new_protected:cpn { \c_keys_props_root_tl .meta:x } #1
  { \keys_meta_make:x {#1} }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{.skip_set:N, .skip_set:c}
% \begin{macro}{.skip_gset:N, .skip_gset:c}
%   Setting a variable is very easy: just pass the data along.
%    \begin{macrocode}
\cs_new_protected_nopar:cpn { \c_keys_props_root_tl .skip_set:N } #1
  { \keys_variable_set:NnN #1 { skip } n }
\cs_new_protected_nopar:cpn { \c_keys_props_root_tl .skip_set:c } #1
  { \keys_variable_set:cnN {#1} { skip } n }
\cs_new_protected_nopar:cpn { \c_keys_props_root_tl .skip_gset:N } #1
  { \keys_variable_set:NnNN #1 { skip } g n }
\cs_new_protected_nopar:cpn { \c_keys_props_root_tl .skip_gset:c } #1
  { \keys_variable_set:cnNN {#1} { skip } g n }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
%
% \begin{macro}{.tl_set:N, .tl_set:c}
% \begin{macro}{.tl_gset:N, .tl_gset:c}
% \begin{macro}{.tl_set_x:N, .tl_set_x:c}
% \begin{macro}{.tl_gset_x:N, .tl_gset_x:c}
%   Setting a variable is very easy: just pass the data along.
%    \begin{macrocode}
\cs_new_protected_nopar:cpn { \c_keys_props_root_tl .tl_set:N } #1
   { \keys_variable_set:NnN #1 { tl } n }
\cs_new_protected_nopar:cpn { \c_keys_props_root_tl .tl_set:c } #1
  { \keys_variable_set:cnN {#1} { tl } n }
\cs_new_protected_nopar:cpn { \c_keys_props_root_tl .tl_set_x:N } #1
  { \keys_variable_set:NnN #1 { tl } x }
\cs_new_protected_nopar:cpn { \c_keys_props_root_tl .tl_set_x:c } #1
  { \keys_variable_set:cnN {#1} { tl } x }
\cs_new_protected_nopar:cpn { \c_keys_props_root_tl .tl_gset:N } #1
  { \keys_variable_set:NnNN #1 { tl } g n }
\cs_new_protected_nopar:cpn { \c_keys_props_root_tl .tl_gset:c } #1
  { \keys_variable_set:cnNN {#1} { tl } g n }
\cs_new_protected_nopar:cpn { \c_keys_props_root_tl .tl_gset_x:N } #1
  { \keys_variable_set:NnNN #1 { tl } g x }
\cs_new_protected_nopar:cpn { \c_keys_props_root_tl .tl_gset_x:c } #1
  { \keys_variable_set:cnNN {#1} { tl } g x }
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
%
% \begin{macro}{.value_forbidden:}
% \begin{macro}{.value_required:}
%   These are very similar, so both call the same function.
%    \begin{macrocode}
\cs_new_protected_nopar:cpn { \c_keys_props_root_tl .value_forbidden: }
  { \keys_value_requirement:n { forbidden } }
\cs_new_protected_nopar:cpn { \c_keys_props_root_tl .value_required: }
  { \keys_value_requirement:n { required } }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \subsection{Setting keys}
%
% \begin{macro}{\keys_set:nn, \keys_set:nV, \keys_set:nv, \keys_set:no}
% \begin{macro}[aux]{\keys_set_aux:nnn, \keys_set_aux:onn}
%   A simple wrapper again.
%    \begin{macrocode}
\cs_new_protected:Npn \keys_set:nn
  { \keys_set_aux:onn { \l_keys_module_tl } }
\cs_new_protected:Npn \keys_set_aux:nnn #1#2#3
  {
    \tl_set:Nn \l_keys_module_tl {#2}
    \keyval_parse:NNn \keys_set_elt:n \keys_set_elt:nn {#3}
    \tl_set:Nn \l_keys_module_tl {#1}
  }
\cs_generate_variant:Nn \keys_set:nn { nV , nv , no }
\cs_generate_variant:Nn \keys_set_aux:nnn { o }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}[int]{\keys_set_elt:n, \keys_set_elt:nn}
% \begin{macro}[aux]{\keys_set_elt_aux:nn}
%   A shared system once again. First, set the current path and add a
%   default if needed. There are then checks to see if the a value is
%   required or forbidden. If everything passes, move on to execute the
%   code.
%    \begin{macrocode}
\cs_new_protected_nopar:Npn \keys_set_elt:n #1
  {
    \bool_set_true:N \l_keys_no_value_bool
    \keys_set_elt_aux:nn {#1} { }
  }
\cs_new_protected:Npn \keys_set_elt:nn #1#2
  {
    \bool_set_false:N \l_keys_no_value_bool
    \keys_set_elt_aux:nn {#1} {#2}
  }
\cs_new_protected:Npn \keys_set_elt_aux:nn #1#2
  {
    \tl_set:Nx \l_keys_key_tl { \tl_to_str:n {#1} }
    \tl_set:Nx \l_keys_path_tl { \l_keys_module_tl / \l_keys_key_tl }
    \keys_value_or_default:n {#2}
    \bool_if:nTF
      {
        \keys_if_value_p:n { required } &&
        \l_keys_no_value_bool
      }
      {
        \msg_kernel_error:nnx { keys } { value-required }
          { \l_keys_path_tl }
      }
      {
        \bool_if:nTF
          {
              \keys_if_value_p:n { forbidden } &&
            ! \l_keys_no_value_bool
          }
          {
            \msg_kernel_error:nnxx { keys } { value-forbidden }
              { \l_keys_path_tl } { \l_keys_value_tl }
          }
          { \keys_execute: }
      }
  }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}[int]{\keys_value_or_default:n}
%   If a value is given, return it as |#1|, otherwise send a default if
%   available.
%    \begin{macrocode}
\cs_new_protected:Npn \keys_value_or_default:n #1
  {
    \tl_set:Nn \l_keys_value_tl {#1}
    \bool_if:NT \l_keys_no_value_bool
      {
        \quark_if_no_value:cF { \c_keys_vars_root_tl \l_keys_path_tl .default }
          {
            \tl_set_eq:Nc \l_keys_value_tl
              { \c_keys_vars_root_tl \l_keys_path_tl .default }
          }
      }
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}[int]{\keys_if_value_p:n}
%   To test if a value is required or forbidden. A simple check for
%   the existence of the appropriate marker.
%    \begin{macrocode}
\prg_new_conditional:Npnn \keys_if_value:n #1 { p }
  {
    \tl_if_eq:ccTF { c_keys_value_ #1 _tl }
      { \c_keys_vars_root_tl \l_keys_path_tl .req }
      { \prg_return_true: }
      { \prg_return_false: }
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}[int]{\keys_execute:}
% \begin{macro}[aux]{\keys_execute_unknown:}
% \begin{macro}[aux]{\keys_execute:nn}
%   Actually executing a key is done in two parts. First, look for the
%   key itself, then look for the \texttt{unknown} key with the same
%   path. If both of these fail, complain.
%    \begin{macrocode}
\cs_new_nopar:Npn \keys_execute:
  { \keys_execute:nn { \l_keys_path_tl } { \keys_execute_unknown: } }
\cs_new_nopar:Npn \keys_execute_unknown:
  {
    \keys_execute:nn { \l_keys_module_tl / unknown }
      {
        \msg_kernel_error:nnxx { keys } { key-unknown }
          { \l_keys_path_tl } { \l_keys_module_tl }
      }
  }
\cs_new_nopar:Npn \keys_execute:nn #1#2
  {
    \cs_if_exist:cTF { \c_keys_code_root_tl #1 }
      {
        \exp_args:Nno \use:c { \c_keys_code_root_tl #1 }
          \l_keys_value_tl
      }
      {#2}
  }
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \end{macro}
%
% \begin{macro}{\keys_choice_find:n}
%   Executing a choice has two parts. First, try the choice given, then
%   if that fails call the unknown key. That will exist, as it is created
%   when a choice is first made. So there is no need for any escape code.
%    \begin{macrocode}
\cs_new_nopar:Npn \keys_choice_find:n #1
  {
    \keys_execute:nn { \l_keys_path_tl / \tl_to_str:n {#1} }
      { \keys_execute:nn { \l_keys_path_tl / unknown } { } }
  }
%    \end{macrocode}
% \end{macro}
%
% \subsection{Utilities}
%
% \begin{macro}[pTF]{\keys_if_exist:nn}
% A utility for others to see if a key exists.
%    \begin{macrocode}
\prg_new_conditional:Npnn \keys_if_exist:nn #1#2 { p , T , F , TF }
  {
    \cs_if_exist:cTF { \c_keys_code_root_tl #1 / #2 }
      { \prg_return_true:}
      { \prg_return_false:}
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\keys_show:nn}
%   Showing a key is just a question of using the correct name.
%    \begin{macrocode}
\cs_new_nopar:Npn \keys_show:nn #1#2
  { \cs_show:c { \c_keys_code_root_tl #1 / \tl_to_str:n {#2} } }
%    \end{macrocode}
% \end{macro}
%
% \subsection{Messages}
%
% For when there is a need to complain.
%    \begin{macrocode}
\msg_kernel_new:nnnn { keys } { boolean-values-only }
  { Key~'#1'~accepts~boolean~values~only. }
  { The~key~'#1'~only~accepts~the~values~'true'~and~'false'. }
\msg_kernel_new:nnnn { keys } { choice-unknown }
  { Choice~'#2'~unknown~for~key~'#1'. }
  {
    The~key~'#1'~takes~a~limited~number~of~values.\\
    The~input~given,~'#2',~is~not~on~the~list~accepted.
  }
\msg_kernel_new:nnnn { keys } { generate-choices-before-code }
  { No~code~available~to~generate~choices~for~key~'#1'. }
  {
    \c_msg_coding_error_text_tl
    Before~using~.generate_choices:n~the~code~should~be~defined~
    with~'.choice_code:n'~or~'.choice_code:x'.
  }
\msg_kernel_new:nnnn { keys } { key-no-property }
  { No~property~given~in~definition~of~key~'#1'. }
  {
    \c_msg_coding_error_text_tl
    Inside~\keys_define:nn each~key~name
    needs~a~property:  \\
    ~ ~ #1 .<property> \\
    LaTeX~did~not~find~a~'.'~to~indicate~the~start~of~a~property.
  }
\msg_kernel_new:nnnn { keys } { key-unknown }
  { The~key~'#1'~is~unknown~and~is~being~ignored. }
  {
    The~module~'#2'~does~not~have~a~key~called~#1'.\\
    Check~that~you~have~spelled~the~key~name~correctly.
  }
\msg_kernel_new:nnnn { keys } { option-unknown }
  { Unknown~option~'#1'~for~package~#2. }
  {
    LaTeX~has~been~asked~to~set~an~option~called~'#1'~
    but~the~#2~package~has~not~created~an~option~with~this~name.
  }
\msg_kernel_new:nnnn { keys } { property-requires-value }
  { The~property~'#1'~requires~a~value. }
  {
    \c_msg_coding_error_text_tl
    LaTeX~was~asked~to~set~property~'#2'~for~key~'#1'.\\
    No~value~was~given~for~the~property,~and~one~is~required.
  }
\msg_kernel_new:nnnn { keys } { property-unknown }
  { The~key~property~'#1'~is~unknown. }
  {
    \c_msg_coding_error_text_tl
    LaTeX~has~been~asked~to~set~the~property~'#1'~for~key~'#2':~
    this~property~is~not~defined.
  }
\msg_kernel_new:nnnn { keys } { value-forbidden }
  { The~key~'#1'~does~not~taken~a~value. }
  {
    The~key~'#1'~should~be~given~without~a~value.\\
    LaTeX~will~ignore~the~given~value~'#2'.
  }
\msg_kernel_new:nnnn { keys } { value-required }
  { The~key~'#1'~requires~a~value. }
  {
    The~key~'#1'~must~have~a~value.\\
    No~value~was~present:~the~key~will~be~ignored.
  }
%    \end{macrocode}
%
%    \begin{macrocode}
%</initex|package>
%    \end{macrocode}
%
%\end{implementation}
%
%\PrintIndex