2>
    <table class="ref-arguments">
    <colgroup><col class="name" /><col class="desc" /></colgroup>
    <tr>
      <th>formula</th>
      <td><p>a formula object, in which the response variable is on the left of a ~ operator, and the groups on the right.</p></td>
    </tr>
    <tr>
      <th>data</th>
      <td><p>a data.frame containing the named variables needed for the formula. If this argument is missing, then the variables in the formula should be on the search list.</p></td>
    </tr>
    <tr>
    