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-<head>
-<title>UK TeX FAQ -- question label labundef</title>
-</head><body>
-<h3>Finding if a label is undefined</h3>
-<p/>People seem to want to know (at run time) if a label is undefined (I
-don&#8217;t actually understand <em>why</em>, particularly: it&#8217;s a transient
-state, and LaTeX deals with it quite well).
-<p/>A resolved label is simply a command:
-<code>\</code><code>r@</code><code>&lt;<i>label-name</i>&gt;</code>; determining if the label is set is
-then simply a matter of detecting if the command exists. The usual
-LaTeX internal way of doing this is to use the command
-<code>\</code><code>@ifundefined</code>:
-<blockquote>
- <code>\</code><code>@ifundefined{</code><em></em>r@<em>label-name</em><code>}{</code><em>undef-cmds</em><code>}{</code><em>def-cmds</em><code>}</code>
-</blockquote><p>
-In which, &lt;<i>label-name</i>&gt; is exactly what you would use in
-a <code>\</code><code>label</code> command, and the remaining two arguments are command
-sequences to be used if the label is undefined
-(&lt;<i>undef-cmds</i>&gt;) or if it is defined
-(&lt;<i>def-cmds</i>&gt;).
-<p/>Note that any command that incorporates <code>\</code><code>@ifundefined</code> is naturally
-fragile, so remember to create it with <code>\</code><code>DeclareRobustCommand</code> or to
-use it with <code>\</code><code>protect</code> in a moving argument.
-<p/>If you&#8217;re into this game, you may well not care about LaTeX&#8217;s
-warning about undefined labels at the end of the document; however,
-if you are, include the command <code>\</code><code>G@refundefinedtrue</code> in
-&lt;<i><code>undef-cmds</i>&gt;</code>.
-<p/>And of course, remember you&#8217;re dealing in internal commands, and pay
-attention to the <a href="FAQ-atsigns.html">at-signs</a>.
-<p/>All the above can be avoided by using the <i>labelcas</i> package:
-it provides commands that enable you to switch according to the state
-of a single label, or the states of a list of labels. The package&#8217;s
-definition is a bit complicated, but the package itself is pretty
-powerful.
-<dl>
-<dt><tt><i>labelcas.sty</i></tt><dd><a href="http://mirror.ctan.org/macros/latex/contrib/labelcas.zip">macros/latex/contrib/labelcas</a> (or <a href="http://mirror.ctan.org/macros/latex/contrib/labelcas/">browse the directory</a>); <a href="http://mirror.ctan.org/help/Catalogue/entries/labelcas.html">catalogue entry</a>
-</dl>
-<p/><p>This question on the Web: <a href="http://www.tex.ac.uk/cgi-bin/texfaq2html?label=labundef">http://www.tex.ac.uk/cgi-bin/texfaq2html?label=labundef</a>
-</body>