      about to be invoked. Say you have a generator <code class="computeroutput"><span class="identifier">G</span></code>,
          and a C++ function <code class="computeroutput"><span class="identifier">F</span></code>, you
          can make the generator call <code class="computeroutput"><span class="identifier">F</span></code>
          just before it gets invoked by attaching <code class="computeroutput"><span class="identifier">F</span></code>:
        </p>
<pre class="programlisting"><span class="i