al">&amp;</span> <span class="identifier">attr</span><span class="special">,</span> <span class="identifier">Context</span><span class="special">&amp;</span> <span class="identifier">context</span><span class="special">,</span> <span class="keyword">bool</span><span class="special">&amp;</span> <span class="identifier">pass</span><span class="special">);</span>
</pre>
<p>
          The function or function object is expected to return the value to generate
          output from by assigning it to the first 