an> <span class="identifier">system</span>
  <span class="special">:</span> <span class="keyword">private</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">base_from_member</span><span class="special">&lt;</span><span class="identifier">an_int</span><span class="special">&gt;</span>
  <span class="special">,</span> <span class="keyword">private</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">base_from_membe