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-<head>
-<title>UK TeX FAQ -- question label patch</title>
-</head><body>
-<h3>Patching existing commands</h3>
-<p/>In the general case (possibly sticking something in the middle of an
-existing command) this is difficult. However, the common requirement,
-to add some code at the beginning or the end of an existing command,
-is conceptually quite easy. Suppose we want to define a version of a
-command that does some small extension of its original definition: we
-would naturally write:
-<blockquote>
-<pre>
-\renewcommand{\splat}{\mumble\splat}
-</pre>
-</blockquote><p>
-However, this would not work: a call to <code>\</code><code>splat</code> would execute
-<code>\</code><code>mumble</code>, and the call the redefined <code>\</code><code>splat</code> again; this is an
-unterminated recursion, that will quickly exhaust TeX&#8217;s memory.
-<p/>Fortunately, the TeX primitive <code>\</code><code>let</code> command comes to our
-rescue; it allows us to take a &#8220;snapshot&#8221; of the current state of a
-command, which we can then use in the redefinition of the command.
-So:
-<blockquote>
-<pre>
-\let\OldSmooth\smooth
-\renewcommand{\smooth}{\mumble\OldSmooth}
-</pre>
-</blockquote><p>
-effects the required patch, safely. Adding things at the end of a
-command works similarly. If <code>\</code><code>smooth</code> takes arguments, one must
-pass them on:
-<blockquote>
-<pre>
-\let\OldSmooth\smooth
-\renewcommand{\smooth}[2]{\mumble\OldSmooth{#1}{#2}}
-</pre>
-
-</blockquote><p>
-<p/>The general case may be achieved in two ways. First, one can use the
-LaTeX command <code>\</code><code>CheckCommand</code>; this compares an existing command
-with the definition you give it, and issues a warning if two don&#8217;t
-match. Use is therefore:
-<blockquote>
- <code>\CheckCommand{\complex}{</code>&lt;<i>original definition</i>&gt;<code>}</code><br>
- <code>\renewcommand{\complex}{</code>&lt;<i>new definition</i>&gt;<code>}</code>
-</blockquote><p>
-This technique is obviously somewhat laborious, but if the original
-command comes from a source that&#8217;s liable to change under the control
-of someone else, it does at least warn you that your patch is in
-danger of going wrong.
-<p/>Otherwise, LaTeX users may use one of the <i>patchcmd</i>,
-<i>ted</i> or <i>etoolbox</i> packages.
-<p/>The <i>patchcmd</i> package addresses a slightly simpler task, by
-restricting the set of commands that you may patch; you mayn&#8217;t patch
-any command that has an optional argument, though it does deal with
-the case of commands defined with <code>\</code><code>DeclareRobustCommand</code>. The
-package defines a <code>\</code><code>patchcommand</code> command, that takes three
-arguments: the command to patch, stuff to stick at the front of its
-definition, and stuff to stick on the end of its definition. So,
-after
-<blockquote>
-<pre>
-\def\b{b}
-\patchcmd\b{a}{c}
-</pre>
-</blockquote><p>
-we will have a new version of <code>\</code><code>b</code> defined as &#8220;<code>abc</code>&#8221;.
-<p/>The <i>ted</i> package is a &#8216;token list editor&#8217;, and provides a
-command <code>\</code><code>substitute</code> which will patch the
-contents of a macro, putting the result in a token-list, or
-(in the form <code>\</code><code>Substitute*</code>) using the result to (re)define a
-macro. The following example defines a simple macro, and then changes
-its definition:
-<blockquote>
-<pre>
-\newcommand{\myfont}%
- {\Large\sffamily\selectfont}
-\Substitute*[\renewcommand{\myfont}]{\myfont}%
- {\Large\sffamily}{\huge\itshape}
-</pre>
-</blockquote><p>
-The macro&#8217;s definition is now:
-<blockquote>
-<pre>
-\huge\itshape\selectfont
-</pre>
-</blockquote><p>
-<p/>The package also offers a command <code>\</code><code>ShowTokens</code>, which shows the
-content of its argument, one token to a line, with details of the
-token&#8217;s category code, etc. This is recommended as a debugging tool.
-<p/>The <i>etoolbox</i> (which provides user access to e-TeX&#8217;s
-programming facilities) provides a command <code>\</code><code>patchcmd</code> which is
-very similar to <code>\</code><code>Substitute</code>, except that it only replaces a
-single instance of the token(s) in its search pattern. Since not all
-commands may be patched in this way, <code>\</code><code>patchcmd</code> takes extra
-arguments for <em>success</em> and <em>failure</em> cases. The
-package also provides commands that prepend (<code>\</code><code>pretocmd</code>) or append
-(<code>\</code><code>apptocmd</code>) to the definition of a command. Not all commands may
-be patched in this way; the package provides a command
-<code>\</code><code>ifpatchable</code> which checks the prerequisites, and checks that the
-target command&#8217;s body does indeed include the patch string you propose
-to use. (The command <code>\</code><code>ifpatchable*</code> omits the test on the patch
-string.)
-<p/>Finally, we&#8217;ll briefly consider a package that is (just about)
-usable, but not really recommendable. <i>Patch</i> gives you an
-ingenious (and difficult to understand) mechanism, and comes as an
-old-style LaTeX documented macro file, which can no longer be
-processed to <a href="FAQ-install-doc.html">produce formatted documentation, etc.</a>.
-Fortunately, the file (<i>patch.doc</i>) may be input directly, and
-&#8220;documentation&#8221; may found by reading the source of the package.
-Roughly speaking, one gives the command a set of instructions
-analogous to <i>sed</i> substitutions, and it regenerates the
-command thus amended. Unless you can&#8217;t do your job any other way,
-<i>patch</i> is best avoided.
-<dl>
-<dt><tt><i>etoolbox.sty</i></tt><dd><a href="http://mirror.ctan.org/macros/latex/contrib/etoolbox.zip">macros/latex/contrib/etoolbox</a> (or <a href="http://mirror.ctan.org/macros/latex/contrib/etoolbox/">browse the directory</a>); <a href="http://mirror.ctan.org/help/Catalogue/entries/etoolbox.html">catalogue entry</a>
-<dt><tt><i>patch.doc</i></tt><dd><a href="http://mirror.ctan.org/macros/generic/misc/patch.doc">macros/generic/misc/patch.doc</a>; <a href="http://mirror.ctan.org//help/Catalogue/entries/patch.html">catalogue entry</a>
-<dt><tt><i>patchcommand.sty</i></tt><dd><a href="http://mirror.ctan.org/macros/latex/contrib/patchcmd.zip">macros/latex/contrib/patchcmd</a> (or <a href="http://mirror.ctan.org/macros/latex/contrib/patchcmd/">browse the directory</a>); <a href="http://mirror.ctan.org/help/Catalogue/entries/patchcmd.html">catalogue entry</a>
-<dt><tt><i>ted.sty</i></tt><dd><a href="http://mirror.ctan.org/macros/latex/contrib/ted.zip">macros/latex/contrib/ted</a> (or <a href="http://mirror.ctan.org/macros/latex/contrib/ted/">browse the directory</a>); <a href="http://mirror.ctan.org/help/Catalogue/entries/ted.html">catalogue entry</a>
-</dl>
-<p/><p>This question on the Web: <a href="http://www.tex.ac.uk/cgi-bin/texfaq2html?label=patch">http://www.tex.ac.uk/cgi-bin/texfaq2html?label=patch</a>
-</body>