           <td><code>R</code>: <code>reference</code> type</td>
            </tr>

            <tr>
              <td><code>V</code>: <code>value_type</code></td>

              <td><code>x1, x2</code>: objects of type <code>T</code></td>
            </tr>
          </table>
        </td>
      </tr>

      <tr>
        <th>Template</th>

        <th>Operations &amp; Requirements</th>
      </tr>

      <tr valign="baseline">
        <td><code><a name="input_iterator_helper">input_iterator_helper&lt;T,
    