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<head><title>POSITION-EQ.html -- ACL2 Version 3.1</title></head>
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<h2>POSITION-EQ</h2>position of an item in a string or a list, using <code><a href="EQ.html">eq</a></code> as test
<pre>Major Section: <a href="PROGRAMMING.html">PROGRAMMING</a>
</pre><p>
<code>(Position-eq item seq)</code> is the least index (zero-based) of the
element <code>item</code> in the list <code>seq</code>, if in fact <code>item</code> is
an element of <code>seq</code>. Otherwise <code>(position-eq item seq)</code> is <code>nil</code>.
<p>
<code>(Position-eq item lst)</code> is provably the same in the ACL2 logic as
<code>(position item lst)</code> and <code>(position-equal item lst)</code> when <code>lst</code> is a
true list, but it has a stronger <a href="GUARD.html">guard</a> because it uses <code><a href="EQ.html">eq</a></code> for a
more efficient test for whether <code>item</code> is equal to a given member of
<code>lst</code>. Its <a href="GUARD.html">guard</a> requires that <code>lst</code> is a true list, and moreover,
either <code>item</code> is a symbol or <code>lst</code> is a list of symbols.
See <a href="POSITION-EQUAL.html">position-equal</a> and see <a href="POSITION.html">position</a>, which unlike <code>position-eq</code> have
guards that allow the second argument to be a string.
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