rd">const</span> <span class="identifier">state_type</span> <span class="special">&amp;</span><span class="identifier">x</span> <span class="special">)</span> <span class="special">{</span>
                                  <span class="keyword">return</span> <span class="identifier">sum</span> <span class="special">+</span> <span class="identifier">x</span><span class="special">[</span><span class="number">0</span><span class="special">];</span> <span class="special">}</span> <span class="special">);</span