 the return type modifier to get back both the type
      and the value in a two-element tuple for our user-defined subtype just as we
      so for any other type.
    </p>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">vmd</span><span class="special">/</span><span class="identifier">equal</span><span class="special">.</span><span class="identifier">hpp</span><