cial">::</span><span class="identifier">pair</span><span class="special">&lt;</span> <span class="keyword">const</span> <span class="identifier">state_type</span> <span class="special">&amp;,</span> <span class="keyword">double</span> <span class="special">&gt;</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