ellpadding="5" border="1" align="center">
      <caption>
        Iterator Helper Class Templates
      </caption>

      <tr>
        <td colspan="2">
          <table align="center" border="1">
            <caption>
              <em>Key</em>
            </caption>

            <tr>
              <td><code>T</code>: operand type</td>

              <td><code>P</code>: <code>pointer</code> type</td>
            </tr>

            <tr>
              <td><code>D</code>: <code>difference_type</code></td>

   