public function renderAsTable() {
    $html = '';
    foreach ($this->cells as $iRow => $rowCells) {
      $rowHtml = '';
      foreach ($rowCells as $iCol => $cell) {
        $rowHtml .= $cell->render();
      }
      $html .= '    <tr>' . $rowHtml . '</tr>' . "\n";
    }
    return "<table>\n  <tbody>\n" . $html . "  </tbody>\n</table>\n";
  }