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+<head>
+<title>UK TeX FAQ -- question label isdef</title>
+</head><body>
+<h3>Is this command defined?</h3>
+<p/>Macro sets from the earliest days of TeX programming may be
+observed to test whether commands exist by using
+<blockquote>
+<code>\</code><code>ifx</code> csx<code><em>command</em></code> <code>\</code><code>undefined</code> &lt;<i>stuff</i>&gt; ...
+</blockquote><p>
+(which of course actually tests that the command <em>doesn&rsquo;t</em>
+exist). LaTeX programmers can make use of the internal command
+<blockquote>
+ <code>\</code><code>@ifundefined{</code><em>cmd name</em><code>}{</code><em>action1</em><code>}{</code><em>action2</em><code>}</code>
+</blockquote><p>
+which executes <code>action1</code> if the command is undefined, and
+<code>action2</code> if it is defined
+(<em>cmd name</em> is the command name only, omitting the &lsquo;<code>\</code>&rsquo; character).
+<p/>The <code>\</code><code>@ifundefined</code> command is based on the sequence
+<blockquote>
+
+<pre>
+\expandafter \ifx \csname cmd name\endcsname \relax
+</pre>
+</blockquote><p>
+which relies on the way <code>\</code><code>csname</code> works: if the command doesn&rsquo;t
+exist, it simply creates it as an alias for <code>\</code><code>relax</code>.
+<p/>So: what is wrong with these techniques?
+<p/>Using <code>\</code><code>undefined</code> blithely assumes that the command is indeed not
+defined. This isn&rsquo;t entirely safe; one could make the name more
+improbable, but that may simply make it more difficult to spot a
+problem when things go wrong. LaTeX programmers who use the
+technique will typically employ <code>\</code><code>@undefined</code>, adding a single
+level of obscurity.
+<p/>The <code>\</code><code>@ifundefined</code> mechanism has the unfortunate property of
+polluting the name space: each test that turns out undefined adds a
+name to the set TeX is holding, and often all those &ldquo;<code>\</code><code>relax</code>&rdquo;
+names serve no purpose whatever. Even so (sadly) there are places in
+the code of LaTeX where the existence of the <code>\</code><code>relax</code> is relied
+upon, after the test, so we can&rsquo;t get away from <code>\</code><code>@ifundefined</code>
+altogether.
+<p/>David Kastrup offers the (rather tricky)
+<blockquote>
+<pre>
+{\expandafter}\expandafter\ifx \csname cmd name\endcsname\relax ...
+</pre>
+
+</blockquote><p>
+which &ldquo;creates&rdquo; the <code>\</code><code>relax</code>-command inside the group of the first
+<code>\</code><code>expandafter</code>, therefore forgets it again once the test is done.
+The test is about as good as you can do with macros.
+<p/>The <a href="FAQ-etex.html">e-TeX system</a> system comes to our help here: it
+defines two new primitives:
+<ul>
+<li> <code>\</code><code>ifdefined</code>, which tests whether a thing is defined (the
+ negative of comparing with <code>\</code><code>undefined</code>, as it were), and
+<li> <code>\</code><code>ifcsname</code> <code>cmd name</code><code>\</code><code>endcsname</code>, which does the
+ negative of <code>\</code><code>@ifundefined</code> without the <code>\</code><code>relax</code>-command
+ side-effect.
+</ul>
+So, in an e-TeX-based system, the following two conditional clauses do
+the same thing:
+<blockquote>
+<pre>
+\ifdefined\foo
+ \message{\string\foo\space is defined}%
+\else
+ \message{no command \string\foo}%
+\fi
+%
+\ifcsname foo\endcsname
+ \message{\string\foo\space is defined}%
+\else
+ \message{no command \string\foo}%
+\fi
+</pre>
+</blockquote><p>
+However, after using the LaTeX
+<code>\</code><code>@ifundefined{foo}</code>..., the conditionals will detect the
+command as &ldquo;existing&rdquo; (since it has been <code>\</code><code>let</code> to <code>\</code><code>relax</code>);
+so it is important not to mix mechanisms for detecting the state of a
+command.
+<p/>Since most distributions nowadays use e-TeX as their base executable
+for most packages, these two primitives may be expected appear widely
+in new macro packages.
+<p/><p/><p/><p>This question on the Web: <a href="http://www.tex.ac.uk/cgi-bin/texfaq2html?label=isdef">http://www.tex.ac.uk/cgi-bin/texfaq2html?label=isdef</a>
+</body>