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<hr size="1">
<a name="Recur-1"></a>
<h4 class="subsubsection">2.7.9.1 Recur</h4>

<a name="index-meta-1"></a>
<a name="index-recur"></a>
<p>A generalization of the form denoted by <code>meta</code> in <code>silly</code> is
recognized by the virtual machine and allows a slightly more efficient
encoding of recursive applications. An expression <code>recur <var>p</var></code>
has the representation indicated by this theorem,
</p>
<dl compact="compact">
<dt> <em>T15</em></dt>
<dd><p>[[<code>recur</code>]] <code><var>p</var></code> = <code>(((nil,<var>p</var>),nil),nil)</code>
</p></dd>
</dl>

<p>which implies that [[<code>meta</code>]] = [[<code>recur</code>]] <code>(nil,nil)</code>.
</p>
<p>If <code><var>p</var></code> is non-<code>nil</code>, a tree of the form [[<code>recur</code>]]
<code><var>p</var></code> is interpreted as follows. Note that <em>P4</em> is
equivalent to the special case of this property for which <code><var>p</var></code>
is <code>(nil,nil)</code>.
</p>
<dl compact="compact">
<dt> <em>P14</em></dt>
<dd><p>([[<code>recur</code>]] <code><var>p</var></code>) <code><var>x</var></code> = [[<code>meta</code>]] ([[<code>field</code>]] <code><var>p</var></code>) <code><var>x</var></code>
</p></dd>
</dl>

<p>The rationale is that <code>meta</code> would very frequently be composed with
a deconstruction <code>field <var>p</var></code>, so the virtual machine saves some
time and space by allowing the two of them to be encoded in a smaller
tree with the combined meaning.
</p>
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