here '.x' is assumed to be the iterated
  over element of <code>.data</code> (e.g., <code>~ mean(.x)</code>)</p></li>
<li><p>providing a name or position of <code>.data</code> to return (e.g.,
  <code>1</code>, <code>"varname"</code>, etc.)</p></li>
</ol></td>
    </tr>
    <tr>
      <th>...</th>
      <td><p>Other values passed to function call.</p></td>
    </tr>
    </table>
    
    <h2 class="hasAnchor" id="value"><a class="anchor" href="#value"></a>Value</h2>

    <p>A character vector</p>
<p>A numer