% \iffalse meta-comment
%
% Copyright (C) 2022 by Zhang Tingxuan <alphaztx@163.com>
%
% This work 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:
%
%   http://www.latex-project.org/lppl.txt
%
% and version 1.3 or later is part of all distributions of
% LaTeX version 2005/12/01 or later.
%
% This work has the LPPL maintenance status `maintained'.
%
% The Current Maintainer of this work is Zhang Tingxuan.
%
% This work consists of the files fixdif.dtx,
%           and the derived files fixdif.ins,
%                                 fixdif.sty,
%                                 fixdif.pdf,
%                             and README.md.
%
%<*internal>
\iffalse
%</internal>
%
%<*readme>
# The `fixdif` Package

The `fixdif` package provides an interface for defining the style of differential operators.

The basic commands are:

- `\d`: for differential operator "d" in math mode and under-dot accent in text;
- `\partial`: a "fixed" partial differential operator command;
- `\letdif`: an interface for defining new commands for differential operators;
- `\mathdif`: a commmand in math mode, you can use it just like `\mathop`, etc.

See `fixdif.pdf` for more. Happy TeXing!

## License

This work may be distributed and/or modified under the conditions of 
the [LaTeX Project Public License](http://www.latex-project.org/lppl.txt), 
either version 1.3c of this license or (at your option) any later version.

------

Copyright (C) 2022 by Zhang Tingxuan <alphaztx@163.com>.
%</readme>
%
%<*internal>
\fi
\begingroup
  \def\NameOfLaTeXe{LaTeX2e}
\expandafter\endgroup\ifx\NameOfLaTeXe\fmtname\else
\csname fi\endcsname
%</internal>
%
%<*install>
\input docstrip.tex
\keepsilent

\preamble

  Copyright (C) 2022 by Zhang Tingxuan <alphaztx@163.com>

  This work 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:

    http://www.latex-project.org/lppl.txt

  and version 1.3 or later is part of all distributions of
  LaTeX version 2005/12/01 or later.
 
  This work has the LPPL maintenance status `maintained'.
 
  The Current Maintainer of this work is Zhang Tingxuan.

  This work consists of the files fixdif.dtx,
            and the derived files fixdif.ins,
                                  fixdif.sty,
                                  fixdif.pdf,
                              and README.md.

\endpreamble

\generate{
  \usedir{tex/latex/fixdif}
    \file{\jobname.sty}      {\from{\jobname.dtx}{package}}
%</install>
%<*internal>
  \usedir{source/latex/fixdif}
    \file{\jobname.ins}      {\from{\jobname.dtx}{install}}
%</internal>
%<*install>
  \usedir{doc/latex/fixdif}
  \nopreamble\nopostamble
    \file{README.md}         {\from{\jobname.dtx}{readme}}
}

\obeyspaces
\Msg{****************************************************}
\Msg{*                                                  *}
\Msg{* To finish the installation you have to move the  *}
\Msg{* following file into a directory searched by TeX: *}
\Msg{*                                                  *}
\Msg{*     fixdif.sty                                   *}
\Msg{*                                                  *}
\Msg{* The recommended directory is                     *}
\Msg{*   TDS:tex/latex/fixdif                           *}
\Msg{*                                                  *}
\Msg{* To produce the documentation run the file        *}
\Msg{* fixdif.dtx through pdfLaTeX/XeLaTeX/LuaLaTeX.    *}
\Msg{*                                                  *}
\Msg{* Happy TeXing!                                    *}
\Msg{*                                                  *}
\Msg{****************************************************}

\endbatchfile
%</install>
%
%<*internal>
\fi
%</internal>
%
%<*driver>
\ProvidesFile{fixdif.dtx}[2022/7/19 (c) Copyright 2022 by Zhang Tingxuan]
\documentclass{ltxdoc}

\usepackage{xcolor}
\definecolor{titlecolor}{RGB}{0,12,160}
\usepackage{doc}
\usepackage{fancyvrb}
\fvset{xleftmargin=2.5em,fontsize=\small,gobble=2}
\MakeShortVerb|

\usepackage{hologo}
\def\XeTeX{\hologo{XeTeX}}
\def\LuaTeX{\hologo{LuaTeX}}

\def\pkg{\textsf}
\def\opt{\texttt}

\title{The \pkg{fixdif} Package}
\author{Zhang Tingxuan}
\date{2022/7/19\quad Version 1.3a\thanks{\url{https://github.com/AlphaZTX/fixdif}}}

\usepackage{hyperref}
\hypersetup{
  pdftitle   = {The fixdif Package},
  pdfauthor  = {Zhang Tingxuan},
  pdfcreator = {pdfLaTeX/XeLaTeX/LuaLaTeX},
  linkcolor  = black,
  urlcolor   = titlecolor!80!black,
}
\def\pkg{\textsf}

\makeatletter
\renewcommand\section{\@startsection{section}{1}{\z@}%
  {-2.5ex plus -1ex minus -.2ex}%
  {1ex plus .2ex}%
  {\normalfont\Large\bfseries\color{titlecolor}}}
\renewcommand\subsection{\@startsection{subsection}{2}{\z@}%
  {-2ex plus -1ex minus -.2ex}%
  {.5ex plus .2ex}%
  {\normalfont\normalsize\bfseries\color{titlecolor}}}
\renewcommand\paragraph{\@startsection{paragraph}{4}{\z@}%
  {.8ex plus .4ex minus .2ex}%
  {-1em}%
  {\normalfont\normalsize\bfseries\color{titlecolor}}}
\def\UrlAlphabet{%
      \do\a\do\b\do\c\do\d\do\e\do\f\do\g\do\h\do\i\do\j%
      \do\k\do\l\do\m\do\n\do\o\do\p\do\q\do\r\do\s\do\t%
      \do\u\do\v\do\w\do\x\do\y\do\z\do\A\do\B\do\C\do\D%
      \do\E\do\F\do\G\do\H\do\I\do\J\do\K\do\L\do\M\do\N%
      \do\O\do\P\do\Q\do\R\do\S\do\T\do\U\do\V\do\W\do\X%
      \do\Y\do\Z}
\def\UrlDigits{\do\1\do\2\do\3\do\4\do\5\do\6\do\7\do\8\do\9\do\0}
\g@addto@macro{\UrlBreaks}{\UrlOrds}
\g@addto@macro{\UrlBreaks}{\UrlAlphabet}
\g@addto@macro{\UrlBreaks}{\UrlDigits}
\ifdefined\Umathcode
  \usepackage{unicode-math}
  \setmathfont{Latin Modern Math}
\fi
\makeatother

\usepackage{fixdif}

\linespread{1.05}
\parskip6pt plus 2pt minus 1pt
\parindent2.5em

\begin{document}
  \DocInput{\jobname.dtx}
\end{document}
%</driver>
% \fi
% 
% \maketitle
% 
% \begin{abstract}
% The \pkg{fixdif} package redefines the \cs{d} command in \LaTeX{} 
% and provides an interface to define commands for differential operators.
%
% The package is compatible with pdf\TeX, \XeTeX{} and \LuaTeX. 
% Furthermore, the package is compatible with \pkg{unicode-math} package 
% in \XeTeX{} and \LuaTeX.
% \end{abstract}
%
% \tableofcontents
%
% \section{The background}
% It's usually recommended that one should reserve a small skip between 
% the differential operator and the expression before it\footnote{See 
% \url{https://tex.stackexchange.com/questions/14821/whats-the-proper-way-to-typeset-a-differential-operator}.}. 
% Take the following cases as an example:
% \[f(x)\mathrm{d}x\qquad\mbox{and}\qquad f(x)\d x.\]
% We usually consider that the example on the right side is better than the 
% one on the left side. The little skip between $f(x)$ and $\d x$ can be 
% regarded as a symbol of the product of $f(x)$ and $\d x$. 
%
% So some users prefer to define a macro like this:
% \begin{Verbatim}
% \renewcommand\d{\mathop{\mathrm{d}}\!}
% \end{Verbatim}
% This macro works well in ``display math'' and ``text math'', but we still 
% face the following three problems:
% \begin{enumerate}
% \item The skip before ``d'' would still be reserved in ``text fraction'', 
% which is regarded bad. For example, |$\d y/\d x$| produces 
% $\mathrm{d}y/\,\mathrm{d}x$;
%
% \item This |\d| command cannot be used out of math mode. In another word, 
% |\d{o}| would not produce ``\d{o}'' in text;
%
% \item The skip between ``d'' and the expression before it can be regarded 
% as a product operator. A product operator is definitely a binary operator. 
%
% Take |\cdot| ($\cdot$) as an example.
% A binary operator reserves small skips before and after itself when in 
% ``display math'' or ``text math'' such as $x\cdot y$, but the skips will 
% disappear in ``script math'' or ``script script math'' such as $a^{x\cdot y}$. 
% Thus the small skip should also disappear in script, but |$a^{f(x)\d x}$| 
% still produces $a^{f(x)\,\mathrm{d}x}$ but not $a^{f(x)\d x}$.
% \end{enumerate}
% To solve these problems, you can try this package.
%
% \section{Introduction}
% To load this package, write
% \begin{Verbatim}
% \usepackage{fixdif}
% \end{Verbatim}
% in the preamble. In your document, 
% \begin{Verbatim}
% \[ f(x)\d x,\quad\frac{\d y}{\d x},\quad\d y/\d x,\quad a^{y\d x}. \]
% \end{Verbatim}
% will produce
% \[f(x)\d x,\quad \frac{\d y}{\d x},\quad \d y/\d x,\quad a^{y\d x}.\]
%
% \subsection{When using \pkg{unicode-math}}
% If you are using \pkg{unicode-math} package with \XeTeX/\LuaTeX{} 
% in your document, you must pay attention to the following items:
% \begin{itemize}
% \item If you want to use \pkg{amsmath} package, make sure that the 
% \pkg{unicode-math} package is loaded \emph{after} \pkg{amsmath}. 
%
% \item You had better specify the math font through the \cs{setmathfont} 
% command provided by \pkg{unicode-math} in order to avoid bad skip in  
% text fraction like $\d y/\d x$.
%
% \item Load the \pkg{fixdif} package \emph{after} \pkg{unicode-math}.
% \end{itemize}
% Therefore the correct order is
% \begin{Verbatim}
% \usepackage{amsmath}
% \usepackage{unicode-math}
% \setmathfont{...}[...]
% \usepackage{fixdif}
% \end{Verbatim}
%
% \subsection{When using \pkg{hyperref}}
% If you want to use the \pkg{hyperref} package simultaneously, 
% remember to load \pkg{hyperref} \emph{before} the \pkg{fixdif} package, 
% otherwise the \pkg{hyperref} package will cause conflicts.
%
% \subsection{Basic commands and package options}
% \DescribeMacro{\d}
% The \pkg{fixdif} package provides a \cs{d} command for the differential 
% operator ``d'' in math mode. When in text, \cs{d} behaves just like 
% the old \cs{d} command in \LaTeX{} or plain \TeX{} as an 
% accent command. For example, 
% \begin{Verbatim}
% $\d x$ and \d x
% \end{Verbatim}
% will produce ``$\d x$ and \d x''.
%
% \paragraph{Set the font of \cs{d}}
% There are two basic package options to control the \cs{d}'s style in math 
% mode~--- \opt{rm} and \opt{normal}. The default option is \opt{rm}, in which 
% case |$f(x)\d x$| produces $f(x)\d x$. If you chose the \opt{normal} option, 
% for example
% \begin{Verbatim}
% \usepackage[normal]{fixdif}
% \end{Verbatim}
% |$f(x)\d x$| would produces $f(x)\,dx$.
%
% \DescribeMacro{\resetdfont}
% Besides the previous two optional fonts, you can reset the font of 
% differential operator ``d'' through \cs{resetdfont} command in preamble:
% \begin{Verbatim}
% \resetdfont{\mathsf}
% \end{Verbatim}
% then |\d x| will produce $\mathsf{d}x$.
%
% \paragraph{Control the behavior of \cs{partial}}
% \DescribeMacro{\partial}
% In default, \cs{partial} will also be regarded as a differential operator 
% in this package. If you don't like this default setting, you can use the 
% \opt{nopartial} option:
% \begin{Verbatim}
% \usepackage[nopartial]{fixdif}
% \end{Verbatim}
%
% \section{Define commands for differential operators}
% \noindent\emph{Attention! The commands in this section 
% can be used in preamble only!}
%
% \subsection{Define commands with a single command name}
% \noindent\textcolor{titlecolor}{\cs{letdif}\marg{cmd}\marg{csname}}\DescribeMacro{\letdif}
% \hfill(preamble only)
%
% The \cs{letdif} command has two arguments~--- the first is the newly-defined 
% command and the second is the control sequence \emph{name} of a math character, 
% that is, a command without its backslash. For example,
% \begin{Verbatim}
% \letdif{\vr}{delta}
% \end{Verbatim}
% then |\vr| will produce a $\delta$ (|\delta|) with automatic skip 
% before it.
%
% Through the \cs{letdif} command, we can redefine a math character command 
% by its name. For example, 
% \begin{Verbatim}
% \letdif{\delta}{delta}
% \end{Verbatim}
% then |\delta| itself will be a differential operator.
%
% The second argument \meta{csname} of \cs{letdif} command can be used 
% repeatedly.
%
% \noindent\textcolor{titlecolor}{\cs{letdif*}\marg{cmd}\marg{csname}}\DescribeMacro{\letdif*}
% \hfill(preamble only)
%
% This command is basically the same as \cs{letdif}, but this command will 
% patch a correction after the differential operator. This is very useful 
% when a math font is setted through \pkg{unicode-math} package. For example, 
% \begin{Verbatim}
% \usepackage{unicode-math}
% \setmathfont{TeX Gyre Termes Math}
% \usepackage{fixdif}
% \letdif{\vr}{updelta}
% \end{Verbatim}
% this will cause bad negative skip after |\vr|, but if you change the last 
% line into
% \begin{Verbatim}
% \letdif*{\vr}{updelta}
% \end{Verbatim}
% you will get the result correct.
%
% \subsection{Define commands with multi commands or a string}
%
% \noindent\textcolor{titlecolor}{\cs{newdif}\marg{cmd}\marg{multi-cmd}}\DescribeMacro{\newdif}
% \hfill(without correction, preamble only)\\
% \noindent\textcolor{titlecolor}{\cs{newdif*}\marg{cmd}\marg{multi-cmd}}\DescribeMacro{\newdif*}
% \hfill(with correction, preamble only)
%
% The first argument of these commands is the newly-defined command; and the 
% second argument should contain \emph{more than one} tokens.
% For example, if you have loaded the \pkg{xcolor} package, 
% you can use the following line:
% \begin{Verbatim}
% \newdif{\redsfd}{\textsf{\color{red}d}}
% \end{Verbatim}
% Then you get the |\redsfd| as a differential operator. Take another example,
% \begin{Verbatim}
% \newdif{\D}{\mathrm{D}}
% \end{Verbatim}
% Then you get |\D| for an uppercase upright ``D'' as a differential operator.
%
% If your second argument contains only one command like |\Delta|, 
% it's recommended to use \cs{letdif} or \cs{letdif*} instead.
%
% \cs{newdif} and \cs{newdif*} will check whether \meta{cmd} has been defined 
% already. If so, an error message will be given.
%
% \noindent\textcolor{titlecolor}{\cs{renewdif}\marg{cmd}\marg{multi-cmd}}\DescribeMacro{\renewdif}
% \hfill(without correction, preamble only)\\
% \noindent\textcolor{titlecolor}{\cs{renewdif*}\marg{cmd}\marg{multi-cmd}}\DescribeMacro{\renewdif*}
% \hfill(with correction, preamble only)
%
% These two commands are basically the same as \cs{newdif} and \cs{newdif*}. 
% The only difference is that \cs{renewdif} and \cs{renewdif*} will check 
% whether \meta{cmd} has \emph{not} been defined yet. If so, an error message 
% will be given.
%
% \section{Using differential operators temporarily}
%
% \noindent\textcolor{titlecolor}{\cs{mathdif}\marg{symbol}}\DescribeMacro{\mathdif}
% \hfill(without correction, in math mode only)\\ 
%\noindent\textcolor{titlecolor}{\cs{mathdif*}\marg{symbol}}\DescribeMacro{\mathdif*}
% \hfill(with correction, in math mode only)
%
% These two commands can be used in math mode only, more specifically, 
% after \verb|\begin{document}|. For example, \verb|$x\mathdif{\Delta}\psi$| 
% will get $x\mathdif{\Delta}\psi$.
%
% \section{Examples}
%
% This section shows how to use this package properly in your document.
%
% Take the two examples below:
% \begin{Verbatim}
% \letdif{\Delta}{Delta}       % Example 1, in preamble
% \letdif{\nabla}{nabla}       % Example 2, in preamble
% \end{Verbatim}
% Actually, the second example is more reasonable. 
% Sometimes, we take ``$\Delta$'' as laplacian (equivalent to $\nabla^2$), 
% while ``$\Delta$'' can also be regarded as a variable 
% or function at some other times. Consequently, it's better to save 
% a different command for ``$\Delta$'' as laplacian while reserve 
% \verb|\Delta| as a command for an ordinary math symbol ``$\Delta$''. 
% However, in the vast majority of cases, ``$\nabla$'' is regarded as 
% nabla operator so there is no need to save a different command for 
% ``$\nabla$''. Then we can correct the code above:
% \begin{Verbatim}
% \letdif{\laplacian}{Delta}   % Example 1, corrected, in preamble
% \end{Verbatim}
% With the \pkg{xparse} package, we can define the command in another method:
% \begin{Verbatim}
% \letdif{\nabla}{nabla}
% \DeclareDocumentCommand{ \laplacian }{ s }{
%   \IfBooleanTF{#1}{\mathdif{\Delta}}{\nabla^2}
% }
% \end{Verbatim}
% Then \verb|\laplacian| produces $\nabla^2$ and \verb|\laplacian*| 
% produces $\Delta$.
%
% \paragraph{Dealing with ``$+$'' and ``$-$''}
% If you input \verb|$-\d x$|, you'll get ``$-\d x$'' in your document. 
% However, if you think ``$-{\d x}$'' is better, you can input 
% \verb|-{\d x}|. The ``\verb|\d x|'' in a \emph{group} will be 
% regarded \textsl{ordinary} but not \textsl{inner} so that the 
% small skip will disappear. Maybe ``$-\d x$'' is just okay.
%
% \section{The source code}
%    \begin{macrocode}
%<*package>
%    \end{macrocode}
% Check the \TeX{} format and provides the package name.
%    \begin{macrocode}
\NeedsTeXFormat{LaTeX2e}
\ProvidesPackage{fixdif}[2022/7/19 Interface for defining differential operators.]
%    \end{macrocode}
% 
% \subsection{Control the skip between slashes and differential operator}
% Change the math code of slash ($/$) and backslash ($\backslash$) so that the skip 
% between slashes and differential operators can be ignored.
%    \begin{macrocode}
\@ifpackageloaded{unicode-math}{
%    \end{macrocode}
% If the \pkg{unicode-math} package has been loaded, use the \XeTeX/\LuaTeX{} primitive 
% \cs{Umathcode} to change the type of slashes. The numeral ``4'' stands for ``open''.
%    \begin{macrocode}
  \Umathcode`\/="4 "0 "002F
  \Umathcode"2044="4 "0 "2044
  \Umathcode"2215="4 "0 "2215
  \Umathcode"2F98="4 "0 "2F98
  \Umathcode`\\="4 "0 "005C
  \Umathcode"2216="4 "0 "2216
  \Umathcode"29F5="4 "0 "29F5
  \Umathcode"29F9="4 "0 "29F9
}{
%    \end{macrocode}
% If the \pkg{unicode-math} package has not been loaded, use the \TeX{} primitive 
% \cs{mathcode} to change the type of slashes. The \cs{backslash} needs to be redefined 
% through \cs{delimiter} primitive too.
%    \begin{macrocode}
  \mathcode`\/="413D
  \mathcode`\\="426E % \backslash
  \def\backslash{\delimiter"426E30F\relax}
}
%    \end{macrocode}
% 
% \subsection{Patch the skips around the differential operator}
% \DescribeMacro{\mup@tch}
% The following \cs{mup@tch} patches the skip after the differential operator.
%    \begin{macrocode}
\def\mup@tch{\mathchoice{\mskip-\thinmuskip}{\mskip-\thinmuskip}{}{}}
%    \end{macrocode}
% The \cs{s@beforep@tch} patches the commands with star (\cs{letdif*}, etc).
%    \begin{macrocode}
\def\s@beforep@tch{\mathchoice{}{}{\mbox{}}{\mbox{}}}
%    \end{macrocode}
% 
% \subsection{Declare the package options}
% Declare the options of the package and execute them.
%    \begin{macrocode}
\DeclareOption{rm}{\@ifpackageloaded{unicode-math}
  {\def\@@dif{\symrm{d}}}{\def\@@dif{\mathrm{d}}}}
\DeclareOption{normal}{\def\@@dif{d}}
\DeclareOption{partial}{\def\fixdif@partial@bool{1}}
\DeclareOption{nopartial}{\def\fixdif@partial@bool{0}}
\ExecuteOptions{rm,partial}
\ProcessOptions\relax
%    \end{macrocode}
% Control the behavior of \cs{partial}.
%    \begin{macrocode}
\def\fixdif@partial@true{1}
\ifx\fixdif@partial@bool\fixdif@partial@true
  \AtEndOfPackage{\letdif{\partial}{partial}}
\fi
%    \end{macrocode}
% \DescribeMacro{\resetdfont}
% Define the \cs{resetdfont} command.
%    \begin{macrocode}
\gdef\resetdfont#1{\let\@@dif\relax%
  \def\@@dif{#1{d}}}
%    \end{macrocode}
% 
% \subsection{Deal with the \cs{d} command}
% \DescribeMacro{\@dif}
% \cs{@dif} is the differential operator produced by \cs{d} in math mode. 
% Here we prefer \cs{mathinner} to |\mathbin| to make the skip.
%    \begin{macrocode}
\def\@dif{\mathinner{\@@dif}\mup@tch}
%    \end{macrocode}
% \DescribeMacro{\d@accent}
% Restore the \cs{d} command in text by \cs{d@accent} with the \cs{let} 
% primitive.
%    \begin{macrocode}
\let\d@accent\d
%    \end{macrocode}
% \DescribeMacro{\d}
% Redefine the \cs{d} command. In text, we need to expand the stuffs after \cs{d}
%    \begin{macrocode}
\DeclareRobustCommand\d{\ifmmode\@dif\else\expandafter\d@accent\fi}
%    \end{macrocode}
% 
% \subsection{User's interface for defining new differential operators}
% \DescribeMacro{\letdif}\DescribeMacro{\letdif*}
% Define the \cs{letdif} and \cs{letdif*} command. The internal version of \cs{letdif} 
% is \cs{@letdif}, of \cs{letdif*} is \cs{s@letdif}.
%    \begin{macrocode}
\def\@letdif#1#2{\AtBeginDocument{%
%    \end{macrocode}
% |#1| is the final command; |#2| is the ``control sequence name'' of |#1|'s initial definition.
% Here we create a command (\cs{csname}|#2@old|\cs{endcsname}) to restore |#2|.
%    \begin{macrocode}
  \ifcsname #2@old\endcsname\else%
  \expandafter\let\csname #2@old\expandafter\endcsname
    \csname #2\endcsname%
  \fi%
%    \end{macrocode}
% Finally let |#1| be the new command.
%    \begin{macrocode}
  \gdef#1{\mathinner{\csname #2@old\endcsname}\mup@tch}%
}}
%    \end{macrocode}
% The definition of \cs{s@letdif} is similar, but with the patch for negative skips.
%    \begin{macrocode}
\def\s@letdif#1#2{\AtBeginDocument{%
  \ifcsname #2@old\endcsname\else%
  \expandafter\let\csname #2@old\expandafter\endcsname
    \csname #2\endcsname%
  \fi%
  \gdef#1{\mathinner{\s@beforep@tch\csname #2@old\endcsname\mbox{}}\mup@tch}%
}}
\def\letdif{\@ifstar\s@letdif\@letdif}
%    \end{macrocode}
% \DescribeMacro{\newdif}\DescribeMacro{\newdif*}
% Define the \cs{newdif} and \cs{newdif*} commands. |#1| is the final command; |#2| is the ``long'' argument.
%    \begin{macrocode}
\long\def\@newdif#1#2{\AtBeginDocument{%
  \ifdefined#1
    \PackageError{fixdif}{\string#1 is already defined.}
      {Try another command instead of \string#1.}%
  \else
    \long\gdef#1{\mathinner{#2}\mup@tch}%
  \fi%
}}
\long\def\s@newdif#1#2{\AtBeginDocument{%
  \ifdefined#1
  \PackageError{fixdif}{\string#1 is already defined.}
    {Try another command instead of \string#1.}%
  \else
    \long\gdef#1{\s@beforep@tch\mathinner{#2\mbox{}}\mup@tch}%
  \fi%
}}
\def\newdif{\@ifstar\s@newdif\@newdif}
%    \end{macrocode}
% \DescribeMacro{\renewdif}\DescribeMacro{\renewdif*}
% Define the \cs{renewdif} and \cs{renewdif*} commands.
%    \begin{macrocode}
\long\def\@renewdif#1#2{\AtBeginDocument{%
  \ifdefined#1
    \long\gdef#1{\mathinner{#2}\mup@tch}%
  \else
    \PackageError{fixdif}{\string#1 has not been defined yet.}
      {You should use \string\newdif instead of \string\renewdif.}%
  \fi%
}}
\long\def\s@renewdif#1#2{\AtBeginDocument{%
  \ifdefined#1
    \long\gdef#1{\s@beforep@tch\mathinner{#2\mbox{}}\mup@tch}%
  \else
    \PackageError{fixdif}{\string#1 has not been defined yet.}
      {You should use \string\newdif instead of \string\renewdif.}%
  \fi%
}}
\def\renewdif{\@ifstar\s@renewdif\@renewdif}
%    \end{macrocode}
% \subsection{In-document commands: \cs{mathdif} and \cs{mathdif*}}
%    \begin{macrocode}
\def\@mathdif#1{\mathinner{#1}\mup@tch}
\def\s@mathdif#1{\s@beforep@tch\mathinner{#1\mbox{}}\mup@tch}
\DeclareRobustCommand\mathdif{\@ifstar\s@mathdif\@mathdif}
%    \end{macrocode}
% End of the package.
%    \begin{macrocode}
%</package>
%    \end{macrocode}
\endinput