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<h5 class="subsubsection">2.7.13.2 Filter</h5>

<p><a name="index-g_t_0040code_007bfilter_007d-320"></a>Another well known list operation is that which applies a predicate to
every item of a list, and deletes those for which the predicate is
false. For a predicate with virtual code <var>p</var>, such an
application can be coded conveniently in this form,

     <dl>
<dt><em>T23</em><dd>[[<code>filter</code>]] <var>p</var> = <code>((nil,nil),(nil,(</code><var>p</var><code>,nil)))</code>
</dl>

<p class="noindent">which is to say that writing <code>((nil,nil),(nil,(</code><var>p</var><code>,nil)))</code> in
<code>silly</code> is the same as writing <code>filter </code><var>p</var>.

   <p>The virtual machine detects code of this form provided that
<var>p</var> is other than <code>nil</code>, and evaluates it consistently
with the following properties, causing it to have the meaning that it
does.

     <dl>
<dt><em>P26</em><dd>([[<code>filter</code>]] <var>p</var>) <code>nil</code> = <code>nil</code>
<br><dt><em>P27</em><dd>([[<code>filter</code>]] <var>p</var>) <code>(</code><var>x</var><code>,</code><var>y</var><code>)</code> =
([[<code>filter</code>]] <var>p</var>) <var>y</var> if <var>p</var> <var>x</var> = <code>nil</code>
<br><dt><em>P28</em><dd>([[<code>filter</code>]] <var>p</var>) <code>(</code><var>x</var><code>,</code><var>y</var><code>)</code> =
<code>(</code><var>x</var><code>,</code>([[<code>filter</code>]] <var>p</var>) <var>y</var><code>)</code> if <var>p</var> <var>x</var> is a non-<code>nil</code> tree
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