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<head><title>SET-DIFFERENCE-EQUAL.html -- ACL2 Version 3.1</title></head>
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<h2>SET-DIFFERENCE-EQUAL</h2>elements of one list that are not elements of another
<pre>Major Section: <a href="PROGRAMMING.html">PROGRAMMING</a>
</pre><p>
<code>(Set-difference-equal x y)</code> equals a list whose members
(see <a href="MEMBER-EQUAL.html">member-equal</a>) contains the members of <code>x</code> that are not
members of <code>y</code>. More precisely, the resulting list is the same as
one gets by deleting the members of <code>y</code> from <code>x</code>, leaving the
remaining elements in the same order as they had in <code>x</code>.
<p>
The <a href="GUARD.html">guard</a> for <code>set-difference-equal</code> requires both arguments to be
true lists. Essentially, <code>set-difference-equal</code> has the same
functionality as the Common Lisp function <code>set-difference</code>, except
that it uses the <code><a href="EQUAL.html">equal</a></code> function to test membership rather than
<code><a href="EQL.html">eql</a></code>. However, we do not include the function <code>set-difference</code>
in ACL2, because the Common Lisp language does not specify the order
of the elements in the list that it returns.<p>
Also see <a href="SET-DIFFERENCE-EQ.html">set-difference-eq</a> for a semantically equivalent function that
executes more efficiently on lists of symbols.
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