vate</span><span class="special">:</span>
    <span class="keyword">struct</span> <span class="identifier">enabler</span> <span class="special">{};</span>  <span class="comment">// a private type avoids misuse</span>

 <span class="keyword">public</span><span class="special">:</span>
    <span class="identifier">node_iter</span><span class="special">()</span>
      <span class="special">:</span> <span class="identifier">node_iter</span><span class="special">::</span><span class="identifier">iterator_adaptor