ss="keyword">struct</span> <span class="identifier">is_positive_number</span> <span class="special">{</span>
  <span class="keyword">bool</span> <span class="keyword">operator</span><span class="special">()(</span><span class="keyword">int</span> <span class="identifier">x</span><span class="special">)</span> <span class="special">{</span> <span class="keyword">return</span> <span class="number">0</span> <span class="special">&lt;</span> <span class="identifier">x</span><span class="special">;</span> <span 