       </p>
<p>
          The second <code class="computeroutput"><span class="identifier">double_</span></code> parser
          attaches this action:
        </p>
<pre class="programlisting"><span class="identifier">ref</span><span class="special">(</span><span class="identifier">n</span><span class="special">)</span> <span class="special">+=</span> <span class="identifier">_1</span>
</pre>
<p>
          So, subsequent numbers add into <code class="computeroutput"><span class="identifier">n</span></code>.