tt> is a tag type for <tt class="literal"><span class="pre">x</span></tt> for <tt class="literal"><span class="pre">op</span></tt>.
<tt class="literal"><span class="pre">tag::value</span></tt> is <tt class="literal"><span class="pre">x</span></tt>'s <em>conversion rank</em>.</td>
</tr>
</tbody>
</table>
<!-- .......................................................................... -->
<pre class="literal-block">
typedef op_impl&lt;
      op_tag&lt;x&gt;::type
    , op_tag&lt;y&gt;::type
    &gt;::<a href="