span><span class="identifier">exchange</span><span class="special">(</span><span class="identifier">other</span><span class="special">.</span><span class="identifier">head_</span><span class="special">,</span> <span class="keyword">nullptr</span><span class="special">))</span>
  <span class="special">,</span> <span class="identifier">tail_</span><span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">exchange</span><span class="special">(</s