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<head><title>BUTLAST.html -- ACL2 Version 3.1</title></head>
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<h2>BUTLAST</h2>all but a final segment of a list
<pre>Major Section: <a href="PROGRAMMING.html">PROGRAMMING</a>
</pre><p>
<code>(Butlast l n)</code> is the list obtained by removing the last <code>n</code>
elements from the true list <code>l</code>. The following is a theorem
(though it takes some effort, including lemmas, to get ACL2 to prove
it).
<pre>
(implies (and (integerp n)
(<= 0 n)
(true-listp l))
(equal (length (butlast l n))
(if (< n (length l))
(- (length l) n)
0)))
</pre>
For related functions, see <a href="TAKE.html">take</a> and see <a href="NTHCDR.html">nthcdr</a>.
<p>
The <a href="GUARD.html">guard</a> for <code>(butlast l n)</code> requires that <code>n</code> is a nonnegative
integer and <code>lst</code> is a true list.<p>
<code>Butlast</code> is a Common Lisp function. See any Common Lisp
documentation for more information. Note: In Common Lisp the
second argument of <code>butlast</code> is optional, but in ACL2 it is
required.
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