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<h5 class="subsubsection">2.7.9.2 Refer</h5>

<p><a name="index-g_t_0040code_007brefer_007d-300"></a>In the style of recursive programming compelled by the available
<code>meta</code> primitive, a function effectively requires a copy of its own
machine code as its left argument. Bringing about that state of
affairs is an interesting party trick.

   <p><a name="index-g_t_0040code_007bbu_007d-301"></a>If we had a definition of <code>bu</code> in the standard library implying

     <dl>
<dt><em>T16</em><dd>([[<code>bu</code>]] <code>(</code><var>f</var><code>,</code><var>k</var><code>)</code>) <var>x</var> = <var>f</var><code>(</code><var>k</var><code>,</code><var>x</var><code>)</code>
</dl>

<p class="noindent">which for the sake of concreteness can be done like this,

<pre class="example">     bu = (hired compose)(
             left,
             (hired couple)(compose(constant,right),constant identity))
</pre>
   <p class="noindent">then a definition of <code>refer</code> as

<pre class="example">     refer = (hired bu)(identity,identity)
</pre>
   <p class="noindent">would be consistent with the following property of the operator.

     <dl>
<dt><em>P15</em><dd>([[<code>refer</code>]] <var>f</var>) <var>x</var> = <var>f</var><code> (</code><var>f</var><code>,</code><var>x</var><code>)</code>
</dl>

<p class="noindent">The proof, as always, is a matter of routine calculation in the manner
of the section on how <code>avram</code> thinks.

   <p>However, this pattern would occur so frequently in recursively defined
functions as to be a significant waste of space and time. Therefore,
rather than requiring it to be defined in terms of other operations, the
virtual machine specification recognizes a pattern of the form below with
respect to property <em>P15</em>,

     <dl>
<dt><em>T17</em><dd>[[<code>refer</code>]] <var>f</var> = <code>(((</code><var>f</var><code>,nil),nil),nil)</code>
</dl>

<p class="noindent">and takes the property to be true by definition of the operator. A
definition of <code>refer</code> consistent with <em>T17</em> is therefore to
<a name="index-standard-library-302"></a>be found in the standard library, not the definition proposed above.

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