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<a name="Predicates"></a>
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<a name="Predicates-1"></a>
<h3 class="subsection">2.7.11 Predicates</h3>

<a name="index-predicates"></a>
<p>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
<code><var>p</var></code> in programs of the form
<code>conditional(<var>p</var>,(<var>f</var>,<var>g</var>))</code> among other things. In
this example, the predicate is applied to the argument, a result of
<code>(nil,nil)</code> causes <code><var>f</var></code> to be applied to it, and a result
of <code>nil</code> causes <code><var>g</var></code> to be applied to it.
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