plex
          number example, we'll be adding the smarts. There's an accumulator (<code class="computeroutput"><span class="keyword">double</span><span class="special">&amp;</span> <span class="identifier">n</span></code>) that adds the numbers parsed. On a
          successful parse, this number is the sum of all the parsed numbers.
        </p>
<p>
          The first <code class="computeroutput"><span class="identifier">double_</span></code> parser
          attaches this action:
        </p>
<pre class