dius</span><span class="special">)</span>
   <span class="special">{</span> <span class="comment">// Constructor just stores value a to find root of.</span>
   <span class="special">}</span>
   <span class="identifier">T</span> <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">T</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">x</span><span class="special">)</span>
   <span class="special">{</span>
      <span cl