% \iffalse meta-comment
%
%% File: xtemplate.dtx
%
% Copyright (C) 1999 Frank Mittelbach, Chris Rowley, David Carlisle
%           (C) 2004-2010 Frank Mittelbach, The LaTeX Project
%           (C) 2011-2024 The LaTeX 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
%
%    https://www.latex-project.org/lppl.txt
%
% This file is part of the "l3packages bundle" (The Work in LPPL)
% and all files in that bundle must be distributed together.
%
% -----------------------------------------------------------------------
%
% The development version of the bundle can be found at
%
%    https://github.com/latex3/latex3
%
% for those people who are interested.
%
%<*driver>
\documentclass[full]{l3doc}
\usepackage{amstext}
\begin{document}
  \DocInput{\jobname.dtx}
\end{document}
%</driver>
% \fi
%
% \title{^^A
%   The \pkg{xtemplate} package\\ Prototype document functions^^A
% }
%
% \author{^^A
%  The \LaTeX{} Project\thanks
%    {^^A
%      E-mail:
%        \href{mailto:latex-team@latex-project.org}
%          {latex-team@latex-project.org}^^A
%    }^^A
% }
%
% \date{Released 2024-03-14}
%
% \maketitle
%
% \begin{documentation}
%
% There are three broad \enquote{layers} between putting down ideas into
% a source file and ending up with a typeset document. These layers of
% document writing are
% \begin{enumerate}
%   \item authoring of the text with mark-up;
%   \item document layout design;
%   \item implementation (with \TeX{} programming) of the design.
% \end{enumerate}
% We write the text as an author, and we see the visual output of the design
% after the document is generated; the \TeX{} implementation in the middle is
% the glue between the two.
%
% \LaTeX{}'s greatest success has been to standardise a system of mark-up that
% balances the trade-off between ease of reading and ease of writing to suit
% almost all forms of technical writing. It's
% other original strength was a good background in typographical design; while
% the standard \LaTeXe{} classes look somewhat dated now in terms of their
% visual design, their typography is generally sound. (Barring the occasional
% minor faults.)
%
% However, \LaTeXe{} has always lacked a standard approach to customising
% the visual design of a document. Changing the looks of the standard classes
% involved either:
% \begin{itemize}
%   \item Creating a new version of the implementation code of the class and
%     editing it.
%   \item Loading one of the many packages to customise certain elements of
%     the standard classes.
%   \item Loading a completely different document class, such as
%     \cls{KOMA-Script} or \cls{memoir}, that allows easy customisation.
% \end{itemize}
% All three of these approaches have their drawbacks and learning curves.
%
% The idea behind \pkg{xtemplate} is to cleanly separate the three layers
% introduced at the beginning of this section, so that document authors who
% are not programmers can easily change the design of their documents.
% \pkg{xtemplate} also makes it easier for \LaTeX{} programmers to provide
% their own customisations on top of a pre-existing class.
%
% \section{What is a document?}
%
% Besides the textual content of the words themselves, the source file
% of a document contains mark-up elements that add structure to the
% document. These elements include sectional divisions, figure/table
% captions, lists of various sorts, theorems/proofs, and so on.
% The list will be different for every document that can be written.
%
% Each element can be represented logically without worrying about the
% formatting, with mark-up such as \cs{section}, \cs{caption},
% |\begin{enumerate}| and so on. The output of each one of these
% document elements will be a typeset representation of the information
% marked up, and the visual arrangement and design of these elements
% can vary widely in producing a variety of desired outcomes.
%
% For each type of document element, there may be design variations that
% contain the same sort of information but present it in slightly
% different ways. For example, the difference between a numbered and an
% unnumbered section, \cs{section} and |\section*|, or the difference
% between an itemised list or an enumerated list.
%
% There are three distinct layers in the definition of
% \enquote{a document} at this level
% \begin{enumerate}
%   \item semantic elements such as the ideas of sections and lists;
%   \item a set of design solutions for representing these elements
%     visually;
%   \item specific variations for these designs that represent the
%     elements in the document.
% \end{enumerate}
% In the parlance of the template system, these are called object types,
% templates, and instances, and they are discussed below in sections
% \ref{sec:objects}, \ref{sec:templates}, and~\ref{sec:instances},
% respectively.
%
% \section {Objects, templates, and instances}
%
% By formally declaring documents to be composed of mark-up elements
% grouped into objects, which are interpreted and typeset with a set of
% templates, each of which has one or more instances with which to
% compose each and every semantic unit of the text, we can cleanly
% separate the components of document construction.
%
% All of the structures provided by the template system are global,
% and do not respect \TeX{} grouping.
%
% \section{Object types}
% \label{sec:objects}
%
% An \emph{object type} (sometimes just \enquote{object}) is an
% abstract idea of a document element that takes a fixed number of
% arguments corresponding to the information from the document author
% that it is representing. A sectioning object, for example, might take
% three inputs: \enquote{title}, \enquote{short title}, and
% \enquote{label}.
%
% Any given document class will define which object types are to be
% used in the document, and any template of a given object type can be
% used to generate an instance for the object. (Of course, different
% templates will produce different typeset representations, but the
% underlying content will be the same.)
%
% \begin{function}{\DeclareObjectType}
%   \begin{syntax}
%     \cs{DeclareObjectType} \Arg{object type} \Arg{no. of args}
%   \end{syntax}
%  This function defines an \meta{object type} taking
%  \meta{number of arguments}, where the \meta{object type} is an
%  abstraction as discussed above. For example,
%   \begin{verbatim}
%     \DeclareObjectType{sectioning}{3}
%   \end{verbatim}
%  creates an object type \enquote{sectioning}, where each use of that
%  object type will need three arguments.
% \end{function}
%
% \section{Templates}
% \label{sec:templates}
%
% A \emph{template} is a generalised design solution for representing
% the information of a specified object type. Templates that do the same
% thing, but in different ways, are grouped together by their object type
% and given separate names. There are two important parts to a template:
% \begin{itemize}
%   \item the parameters it takes to vary the design it is producing;
%   \item the implementation of the design.
% \end{itemize}
% As a document author or designer does not care about the
% implementation but rather only the interface to the template, these two
% aspects of the template definition are split into two independent
% declarations, \cs{DeclareTemplateInterface} and
% \cs{DeclareTemplateCode}.
%
% \begin{function}{\DeclareTemplateInterface}
%   \begin{syntax}
%     \cs{DeclareTemplateInterface}
%     ~~\Arg{object type} \Arg{template} \Arg{no.~of args}
%     ~~\Arg{key list}
%   \end{syntax}
%   A \meta{template} interface is declared for a particular
%   \meta{object type}, where the \meta{number of arguments} must
%   agree with the object type declaration. The interface itself is
%   defined by the \meta{key list}, which is itself a key--value list
%   taking a specialized format:
%   \begin{quotation}
%     \obeylines
%     \noindent
%      \meta{key1}~":"~\meta{key type1}~","
%      \meta{key2}~":"~\meta{key type2}~","
%      \meta{key3}~":"~\meta{key type3}~"="~\meta{default3}~","
%      \meta{key4}~":"~\meta{key type4}~"="~\meta{default4}~","
%      \ldots
%    \end{quotation}
%   Each \meta{key} name should consist of \textsc{ascii} characters,
%   with the exception of |,|, |=| and \verb*| |. The recommended form
%   for key names is to use lower case letters, with dashes to separate
%   out different parts. Spaces are ignored in key names, so they can be
%   included or missed out at will. Each \meta{key} must have a
%   \meta{key type}, which defined the type of input that the \meta{key}
%   requires. A full list of key types is given in
%   Table~\ref{tab:key-types}.  Each key may have a \meta{default}
%   value, which will be used in by the template if the \meta{key} is
%   not set explicitly. The \meta{default} should be of the correct
%   form to be accepted by the \meta{key type} of the \meta{key}: this
%   is not checked by the code.
% \end{function}
%
%   \begin{table}
%     \centering
%     \begin{tabular}{>{\ttfamily}ll}
%       \toprule
%       \multicolumn{1}{l}{Key-type} & Description of input \\
%       \midrule
%       boolean    & \texttt{true} or \texttt{false}            \\
%       choice\Arg{choices}
%         & A list of pre-defined \meta{choices} \\
%       commalist  & A comma-separated list                        \\
%       function\Arg{$N$}
%         & A function definition with $N$ arguments
%          ($N$ from $0$ to $9$) \\
%       instance\Arg{name}
%         & An instance of type \meta{name} \\
%       integer    & An integer or integer expression            \\
%       length     & A fixed length                              \\
%       muskip     & A math length with shrink and stretch components \\
%       real       & A real (floating point) value               \\
%       skip       & A length with shrink and stretch components \\
%       tokenlist  & A token list: any text or commands          \\
%       \bottomrule
%     \end{tabular}
%     \caption{Key-types for defining template interfaces with
%       \cs{DeclareTemplateInterface}.}
%     \label{tab:key-types}
%   \end{table}
%
% \begin{function}{\KeyValue}
%   \begin{syntax}
%     \cs{KeyValue} \Arg{key name}
%   \end{syntax}
%   There are occasions where the default (or value) for one key
%   should be taken from another. The \cs{KeyValue} function can be
%   used to transfer this information without needing to know the
%   internal implementation of the key:
%   \begin{verbatim}
%     \DeclareTemplateInterface { object } { template } { no. of args }
%       {
%         key-name-1 : key-type = value ,
%         key-name-2 : key-type = \KeyValue { key-name-1 },
%         ...
%       }
%   \end{verbatim}
% \end{function}
%
% \begin{function}{\DeclareTemplateCode}
%   \begin{syntax}
%     \cs{DeclareTemplateCode}
%     ~~\Arg{object type} \Arg{template} \Arg{no.~of args}
%     ~~\Arg{key bindings} \Arg{code}
%   \end{syntax}
%   The relationship between a templates keys and the internal
%   implementation is created using the \cs{DeclareTemplateCode}
%   function. As with \cs{DeclareTemplateInterface}, the
%   \meta{template} name is given along with the \meta{object type}
%   and \meta{number of arguments} required. The \meta{key bindings}
%   argument is a key--value list which specifies the relationship
%   between each \meta{key} of the template interface with an
%   underlying\meta{variable}.
%
%   \begin{quotation}
%     \obeylines
%     \noindent
%     \meta{key1}~"="~\meta{variable1},
%     \meta{key2}~"="~\meta{variable2},
%     \meta{key3}~"="~global~\meta{variable3},
%     \meta{key4}~"="~global~\meta{variable4},
%     \ldots
%   \end{quotation}
%   With the exception of the choice, code and function key types,
%   the \meta{variable} here should be the name of an existing
%   \LaTeX3 register. As illustrated, the key word \enquote{global}
%   may be included in the listing to indicate that the \meta{variable}
%   should be assigned globally. A full list of variable bindings is
%   given in Table~\ref{tab:key-vars}.
%
%   The \meta{code} argument of \cs{DeclareTemplateCode} is used
%   as the replacement text for the template when it is used, either
%   directly or as an instance. This may therefore accept arguments
%   |#1|, |#2|, \emph{etc}.~as detailed by the \meta{number of arguments}
%   taken by the object type.
% \end{function}
%
%   \begin{table}
%     \centering
%     \begin{tabular}{>{\ttfamily}ll}
%       \toprule
%       \multicolumn{1}{l}{Key-type} & Description of binding \\
%       \midrule
%       boolean    & Boolean variable, \emph{e.g}.~\cs{l_tmpa_bool}      \\
%       choice
%         & List of choice implementations
%         (see Section~\ref{sec:choices-key}) \\
%       commalist  & Comma list, \emph{e.g}.~\cs{l_tmpa_clist}           \\
%       function
%         & Function taking $N$ arguments, \emph{e.g}.~\cs{use_i:nn}     \\
%       instance \\
%       integer    & Integer variable, \emph{e.g}.~\cs{l_tmpa_int}       \\
%       length     & Dimension variable, \emph{e.g}.~\cs{l_tmpa_dim}     \\
%       muskip     & Muskip variable, \emph{e.g}.~\cs{l_tmpa_muskip}     \\
%       real       & Floating-point variable, \emph{e.g}.~\cs{l_tmpa_fp} \\
%       skip       & Skip variable, \emph{e.g}.~\cs{l_tmpa_skip}         \\
%       tokenlist  & Token list variable, \emph{e.g}.~\cs{l_tmpa_tl}     \\
%       \bottomrule
%     \end{tabular}
%     \caption{Bindings required for different key types when defining
%       template implementations with \cs{DeclareTemplateCode}. Apart
%       from \texttt{code}, \texttt{choice} and \texttt{function}
%       all of these accept the key word \texttt{global} to carry
%       out a global assignment.}
%     \label{tab:key-vars}
%   \end{table}
%
% \begin{function}{\AssignTemplateKeys}
%   \begin{syntax}
%     \cs{AssignTemplateKeys}
%   \end{syntax}
%   In the final argument of \cs{DeclareTemplateCode} the assignment of
%   keys defined by the template may be delayed by including the command
%   \cs{AssignTemplateKeys}. If this is \emph{not} present, keys are assigned
%   immediately before the template code. If \cs{AssignTemplateKeys} is
%   present, assignment is delayed until this point. Note that the
%   command must be \emph{directly} present in the code, not placed
%   within a nested command/macro.
% \end{function}
%
% \section{Multiple choices}
% \label{sec:choices-key}
%
% The \texttt{choice} key type implements multiple choice input. At the
% interface level, only the list of valid choices is needed:
% \begin{verbatim}
%   \DeclareTemplateInterface { foo } { bar } { 0 }
%     { key-name : choice { A, B, C } }
% \end{verbatim}
% where the choices are given as a comma-list (which must therefore
% be wrapped in braces). A default value can also be given:
% \begin{verbatim}
%   \DeclareTemplateInterface { foo } { bar } { 0 }
%     { key-name : choice { A, B, C } = A }
% \end{verbatim}
%
% At the implementation level, each choice is associated with code,
% using a nested key--value list.
% \begin{verbatim}
%   \DeclareTemplateCode { foo } { bar } { 0 }
%     {
%       key-name =
%         {
%           A = Code-A ,
%           B = Code-B ,
%           C = Code-C
%         }
%      }
%      { ... }
% \end{verbatim}
% The two choice lists should match, but in the implementation a
% special \texttt{unknown} choice is also available. This can be used
% to ignore values and implement an \enquote{else} branch:
% \begin{verbatim}
%   \DeclareTemplateCode { foo } { bar } { 0 }
%     {
%       key-name =
%         {
%           A       = Code-A ,
%           B       = Code-B ,
%           C       = Code-C ,
%           unknown = Else-code
%         }
%      }
%      { ... }
% \end{verbatim}
% The \texttt{unknown} entry must be the last one given, and should
% \emph{not} be listed in the interface part of the template.
%
% For keys which accept the values \texttt{true} and \texttt{false}
% both the boolean and choice key types can be used. As template
% interfaces are intended to prompt clarity at the design level, the
% boolean key type should be favoured, with the choice type reserved
% for keys which take arbitrary values.
%
% \section{Instances}
% \label{sec:instances}
%
% After a template is defined it still needs to be put to use. The
% parameters that it expects need to be defined before it can be used in
% a document. Every time a template has parameters given to it, an
% \emph{instance} is created, and this is the code that ends up in the
% document to perform the typesetting of whatever pieces of information
% are input into it.
%
% For example, a template might say \enquote{here is a section with or
% without a number that might be centred or left aligned and print its
% contents in a certain font of a certain size, with a bit of a gap
% before and after it} whereas an instance declares \enquote{this is a
% section with a number, which is centred and set in $12\,\text{pt}$
% italic with a $10\,\text{pt}$ skip before and a
% $12\,\text{pt}$ skip after it}. Therefore, an instance is just a
% frozen version of a template with specific settings as chosen by the
% designer.
%
% \begin{function}{\DeclareInstance}
%   \begin{syntax}
%     \cs{DeclareInstance}
%     ~~\Arg{object type} \Arg{instance} \Arg{template} \Arg{parameters}
%   \end{syntax}
%   This function uses a \meta{template} for an \meta{object type}
%   to create an \meta{instance}. The \meta{instance} will be set
%   up using the \meta{parameters}, which will set some of the
%   \meta{keys} in the \meta{template}.
%
%   As a practical example, consider an object type for document sections
%   (which might include chapters, parts, sections, \emph{etc}.), which
%   is called \texttt{sectioning}. One possible template for this
%   object type might be called \texttt{basic}, and one instance of this
%   template would be a numbered section. The instance declaration might
%   read:
%   \begin{verbatim}
%     \DeclareInstance { sectioning } { section-num } { basic }
%       {
%         numbered      = true ,
%         justification = center ,
%         font          =\normalsize\itshape ,
%         before-skip   = 10pt ,
%         after-skip    = 12pt ,
%       }
%   \end{verbatim}
%   Of course, the key names here are entirely imaginary, but illustrate
%   the general idea of fixing some settings.
% \end{function}
%
% \begin{function}{\IfInstanceExistT, \IfInstanceExistF, \IfInstanceExistTF}
%   \begin{syntax}
%     \cs{IfInstanceExistTF} \Arg{object type} \Arg{instance} \Arg{true code} \Arg{false code}
%   \end{syntax}
%   Tests if the named \meta{instance} of a \meta{object type} exists, and
%   then inserts the appropriate code into the input stream.
% \end{function}
%
% \begin{function}{\DeclareInstanceCopy}
%   \begin{syntax}
%     \cs{DeclareInstanceCopy}
%     ~~\Arg{object type} \Arg{instance2} \Arg{instance1}
%   \end{syntax}
%   Copies the \meta{values} for \meta{instance1} for an
%   \meta{object type} to \meta{instance2}.
% \end{function}
%
% \section{Document interface}
%
% After the instances have been chosen, document commands must be
% declared to use those instances in the document. \cs{UseInstance}
% calls instances directly, and this command should be used internally
% in document-level mark-up.
%
% \begin{function}{\UseInstance}
%   \begin{syntax}
%     \cs{UseInstance}
%     ~~\Arg{object type} \Arg{instance} \meta{arguments}
%   \end{syntax}
%   Uses an \meta{instance} of the \meta{object type}, which will require
%   \meta{arguments} as determined by the number specified for the
%   \meta{object type}. The \meta{instance} must have been declared
%   before it can be used, otherwise an error is raised.
% \end{function}
%
% \begin{function}{\UseTemplate}
%   \begin{syntax}
%     \cs{UseTemplate} \Arg{object type} \Arg{template}
%     ~~\Arg{settings} \meta{arguments}
%   \end{syntax}
%   Uses the \meta{template} of the specified \meta{object type},
%   applying the \meta{settings} and absorbing \meta{arguments} as
%   detailed by the \meta{object type} declaration. This in effect
%   is the same as creating an instance using \cs{DeclareInstance}
%   and immediately using it with \cs{UseInstance}, but without the
%   instance having any further existence. It is therefore useful where
%   a template needs to be used once.
%
%   This function can also be used as the argument to \texttt{instance}
%   key types:
%   \begin{verbatim}
%     \DeclareInstance { object } { template } { instance }
%       {
%         instance-key =
%           \UseTemplate { object2 } { template2 } { <settings> }
%       }
%   \end{verbatim}
% \end{function}
%
% \section{Changing existing definitions}
%
% Template parameters may be assigned specific defaults for instances
% to use if the instance declaration doesn't explicit set those
% parameters. In some cases, the document designer will wish to edit
% these defaults to allow them to \enquote{cascade} to the instances.
% The alternative would be to set each parameter identically for each
% instance declaration, a tedious and error-prone process.
%
% \begin{function}{\EditTemplateDefaults}
%   \begin{syntax}
%     \cs{EditTemplateDefaults}
%     ~~\Arg{object type} \Arg{template} \Arg{new defaults}
%   \end{syntax}
%   Edits the \meta{defaults} for a \meta{template} for an
%   \meta{object type}. The \meta{new defaults}, given as a key--value
%   list, replace the existing defaults for the \meta{template}. This
%   means that the change will apply to instances declared after the
%   editing, but that instances which have already been created are
%   unaffected.
% \end{function}
%
% \begin{function}{\EditInstance}
%   \begin{syntax}
%     \cs{EditInstance}
%     ~~\Arg{object type} \Arg{instance} \Arg{new values}
%   \end{syntax}
%   Edits the \meta{values} for an \meta{instance} for an
%   \meta{object type}. The \meta{new values}, given as a key--value
%   list, replace the existing values for the \meta{instance}. This
%   function is complementary to \cs{EditTemplateDefaults}:
%   \cs{EditInstance} changes a single instance while leaving the
%   template untouched.
% \end{function}
%
% \section{When template parameters should be frozen}
%
% A class designer may be inheriting templates declared by someone else,
% either third-party code or the \LaTeX{} kernel itself. Sometimes these
% templates will be overly general for the purposes of the document.
% The user should be able to customise parts of the template instances,
% but otherwise be restricted to only those parameters allowed by the
% designer.
%
% \begin{function}{\DeclareRestrictedTemplate}
%   \begin{syntax}
%     \cs{DeclareRestrictedTemplate}
%     ~~\Arg{object type} \Arg{parent template} \Arg{new template}
%     ~~\Arg{parameters}
%   \end{syntax}
%   Creates a copy of the \meta{parent template} for the
%   \meta{object type} called \meta{new template}. The key--value list
%   of \meta{parameters} applies in the \meta{new template} and cannot
%   be changed when creating an instance.
% \end{function}
%
% \section{\emph{Ad hoc} adjustment of templates}
%
% \begin{function}{\SetTemplateKeys}
%   \begin{syntax}
%    \cs{SetTemplateKeys} \Arg{object type} \Arg{template} \Arg{keyvals}
%   \end{syntax}
%   At point of use it may be useful to apply changed to individual instances.
%   This is supported as each template key is made available for adjustment
%   using \cs{SetTemplateKeys}.
% \end{function}
%
% For example, after
% \begin{verbatim}
%   \DeclareObjectType{MyObj}{0}
%   \DeclareTemplateInterface{MyObj}{TemplateA}{0}
%     {
%       akey: tokenlist  ,
%       bkey: function{2}
%     }
%   \DeclareTemplateCode{MyObj}{TemplateA}{0}
%     {
%       akey = SomeTokens ,
%       bkey = \func:nn ,
%     }
% \end{verbatim}
% the template keys could be adjusted in an \emph{ad hoc} fashion using
% \begin{verbatim}
%   \SetTemplateKeys{MyObj}{TemplateA}
%     {
%       akey = OtherTokens ,
%       bkey = \AltFunc:nn
%     }
% \end{verbatim}
%
% \section{Getting information about templates and instances}
%
% \begin{function}{\ShowInstanceValues}
%   \begin{syntax}
%    \cs{ShowInstanceValues} \Arg{object type} \Arg{instance}
%   \end{syntax}
%   Shows the \meta{values} for an \meta{instance} of the given
%   \meta{object type} at the terminal.
% \end{function}
%
% \begin{function}{\ShowTemplateCode}
%   \begin{syntax}
%     \cs{ShowTemplateCode} \Arg{object type} \Arg{template}
%   \end{syntax}
%   Shows the \meta{code} of a \meta{template} for an \meta{object type}
%   in the terminal.
% \end{function}
%
% \begin{function}{\ShowTemplateDefaults}
%   \begin{syntax}
%     \cs{ShowTemplateDefaults} \Arg{object type} \Arg{template}
%   \end{syntax}
%   Shows the \meta{default} values of a \meta{template} for an
%   \meta{object type} in the terminal.
% \end{function}
%
% \begin{function}{\ShowTemplateInterface}
%   \begin{syntax}
%     \cs{ShowTemplateInterface} \Arg{object type} \Arg{template}
%   \end{syntax}
%  Shows the \meta{keys} and associated \meta{key types} of a
%   \meta{template} for an \meta{object type} in the terminal.
% \end{function}
%
% \begin{function}{\ShowTemplateVariables}
%   \begin{syntax}
%     \cs{ShowTemplateVariables} \Arg{object type} \Arg{template}
%   \end{syntax}
%   Shows the \meta{variables} and associated \meta{keys} of a
%   \meta{template} for an \meta{object type} in the terminal. Note that
%   \texttt{code} and \texttt{choice} keys do not map directly to variables
%   but to arbitrary code. For \texttt{choice} keys, each valid choice
%   is shown as a separate entry in the list, with the key name and choice
%   separated by a space, for example
%   \begin{verbatim}
%     Template 'example' of object type 'example' has variable mapping:
%     >  demo unknown  =>  \def \demo {?}
%     >  demo c  =>  \def \demo {c}
%     >  demo b  =>  \def \demo {b}
%     >  demo a  =>  \def \demo {a}.
%   \end{verbatim}
%   would be shown for a choice key \texttt{demo} with valid choices
%   \texttt{a}, \texttt{b} and \texttt{c}, plus code for an \texttt{unknown}
%   branch.
% \end{function}
%
% \section{Collections}
%
% The implementation of templates includes a concept termed
% \enquote{collections}. The idea is that by activating a collection, a
% set of instances can rapidly be set up. An example use case would be
% collections for \texttt{frontmatter}, \texttt{mainmatter} and
% \texttt{backmatter} in a book. This mechanism is currently implemented
% by the commands \cs{DeclareCollectionInstance}, \cs{EditCollectionInstance}
% and \cs{UseCollection}. However, while the idea of switchable instances is
% a useful one, the team feel that collections are not the correct way to
% achieve this, at least with the current approach. As such, the collection
% functions should be regarded as deprecated: they remain available to support
% existing code, but will be removed when a better mechanism is developed.
%
%
% \begin{function}{\ShowCollectionInstanceValues}
%   \begin{syntax}
%    \cs{ShowInstanceValues} \Arg{collection} \Arg{object type} \Arg{instance}
%   \end{syntax}
%   Shows the \meta{values} for an \meta{instance} within
%   a \meta{collection} of the given \meta{object type} at the terminal.
%   As for other collection commands, this should be regarded as deprecated.
% \end{function}
%
% \end{documentation}
%
% \begin{implementation}
%
% \section{\pkg{xtemplate} implementation}
%
%    \begin{macrocode}
%<*package>
%    \end{macrocode}
%
%    \begin{macrocode}
%<@@=template>
%    \end{macrocode}
%
% \begin{macro}{\DeclareRestrictedTemplate}
% \begin{macro}{\DeclareObjectType}
%   If the new kernel code is not loaded, hand over to the frozen version.
%   Otherwise, make a couple of compatibility additions.
%    \begin{macrocode}
\@ifundefined{NewTemplateType}
  {%
    \input{xtemplate-2023-10-10.sty}%
    \endinput
  }
  {%
    \ProvidesExplPackage{xtemplate}{2024-03-14}{}
      {L3 Experimental prototype document functions}
    \long\protected\def\DeclareRestrictedTemplate#1#2#3#4{%
      \DeclareTemplateCopy{#1}{#3}{#2}%
      \EditTemplateDefaults{#1}{#3}{#4}%
    }%
  }
\ExplSyntaxOn
\cs_new_protected:Npn \DeclareObjectType #1#2
  { \@@_declare_type:nn {#1} {#2} }
\ExplSyntaxOff
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
%</package>
%    \end{macrocode}
%
%\end{implementation}
%
%\PrintIndex
