nt: the
      parsed number. Thus, if we were to attach a function <b>F</b> to
      <tt>real_p</tt>, we need <b>F</b> to be declared as:
  </p>
    
<pre><code>    </code><code><span class=
"keyword">void</span> <span class="identifier">F</span><span class=
"special">(</span><span class="keyword">double</span> <span class=
"identifier">n</span><span class="special">);</span></code></pre>
<p>
      For our example however, again, we can take advantage of some predefined
      semantic functors and functor g