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<h5 class="subsubsection">2.7.9.1 Recur</h5>

<p><a name="index-g_t_0040code_007bmeta_007d-298"></a><a name="index-g_t_0040code_007brecur_007d-299"></a>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 </code><var>p</var>
has the representation indicated by this theorem,

     <dl>
<dt><em>T15</em><dd>[[<code>recur</code>]] <var>p</var> = <code>(((nil,</code><var>p</var><code>),nil),nil)</code>
</dl>

<p class="noindent">which implies that [[<code>meta</code>]] = [[<code>recur</code>]] <code>(nil,nil)</code>.

   <p>If <var>p</var> is non-<code>nil</code>, a tree of the form [[<code>recur</code>]]
<var>p</var> is interpreted as follows. Note that <em>P4</em> is
equivalent to the special case of this property for which <var>p</var>
is <code>(nil,nil)</code>.

     <dl>
<dt><em>P14</em><dd>([[<code>recur</code>]] <var>p</var>) <var>x</var> = [[<code>meta</code>]] ([[<code>field</code>]] <var>p</var>) <var>x</var>
</dl>

   <p>The rationale is that <code>meta</code> would very frequently be composed with
a deconstruction <code>field </code><var>p</var>, 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.

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