an class="identifier">P</span><span class="special">&gt;</span>
    <span class="keyword">bool</span> <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">P</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">p</span><span class="special">)</span> <span class="keyword">const</span>
    <span class="special">{</span>
        <span class="keyword">return</span> <span class="identifier">boost</span><span class="special">: