routput"><span class="identifier">double_</span></code> parser
        attaches this action:
      </p>
<pre class="programlisting"><span class="special">[&amp;](</span><span class="keyword">auto</span><span class="special">&amp;</span> <span class="identifier">ctx</span><span class="special">){</span> <span class="identifier">n</span> <span class="special">+=</span> <span class="identifier">_attr</span><span class="special">(</span><span class="identifier">ctx</span><span class="special">);</span> <span cl