 width="50%"><pre>const T &amp;</pre>
        </td>
    </tr>
    <tr>
        <td valign="top" width="50%"><pre>T &amp;</pre>
        </td>
        <td valign="top" width="50%"><pre>T &amp;</pre>
        </td>
    </tr>
    <tr>
        <td valign="top" width="50%"><pre>const T &amp;</pre>
        </td>
        <td valign="top" width="50%"><pre>const T &amp;</pre>
        </td>
    </tr>
</table>

<p>A little familiarity with the type traits classes allows us to
construct a single mapping that allows us to