g the left and right sides respectively.
        In order to specify a custom tag, the type of each side has to be tagged.
        Tagging a type is very simple:
      </p>
<pre class="programlisting"><span class="keyword">typedef</span> <span class="identifier">tagged</span><span class="special">&lt;</span> <span class="keyword">int</span><span class="special">,</span> <span class="identifier">a_tag</span> <span class="special">&gt;</span> <span class="identifier">tagged_int</span><span class="special">;</