pan class="special">&amp;</span> <span class="identifier">base_reference</span><span class="special">()</span> <span class="keyword">const</span><span class="special">;</span>
    <span class="identifier">Base</span><span class="special">&amp;</span> <span class="identifier">base_reference</span><span class="special">();</span>
 <span class="keyword">private</span><span class="special">:</span> <span class="comment">// Core iterator interface for iterator_facade.  </span>
    <span class="keyword">typename<