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<h4 class="subsection">2.7.11 Predicates</h4>

<p><a name="index-predicates-310"></a>A couple of operations are built into the virtual machine for performing
tests efficiently. These functions return either <code>nil</code> for false or
<code>(nil,nil)</code> for true, and are useful for example as a predicate
<var>p</var> in programs of the form
<code>conditional(</code><var>p</var><code>,(</code><var>f</var><code>,</code><var>g</var><code>))</code> among other things. In
this example, the predicate is applied to the argument, a result of
<code>(nil,nil)</code> causes <var>f</var> to be applied to it, and a result
of <code>nil</code> causes <var>g</var> to be applied to it.

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