EGISTER_ macro, which expands to a tuple whose single element
      is the identifier, the BOOST_VMD_SUBTYPE_REGISTER_ expands to a tuple of two
      elements where the first element is the subtype v-type and the second element
      is the identifier.
    </p>
<p>
      For our udef user-defined subtype this would be:
    </p>
<pre class="programlisting"><span class="preprocessor">#define</span> <span class="identifier">BOOST_VMD_SUBTYPE_REGISTER_UDEF_CIRCLE</span> <span class="special">(</span><span clas