code>.
        </p>
<p>
          The function/function object signature depends on the type of the parser
          to which it is attached. The parser <code class="computeroutput"><span class="identifier">double_</span></code>
          passes the parsed number. Thus, if we were to attach a function <code class="computeroutput"><span class="identifier">F</span></code> to <code class="computeroutput"><span class="identifier">double_</span></code>,
          we need <code class="computeroutput"><span class=