rther from a top-level enclosing type to a deeply nested type, or even to
      look for some deeply nested type's inner elements. We are going to be generating
      a great deal of <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">if_</span></code>
      and/or <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="