public function getHtmlHeader(): string
  {
    $ret = Html::generateTag('tr', ['class' => [OverviewTable::$class, 'header']]);
    foreach ($this->slatJoints as $factory)
    {
      $ret .= $factory->getHtmlColumnHeader();
    }
    $ret .= '<th class="error"></th>';
    $ret .= '</tr>';

    if ($this->filter)
    {
      $ret .= Html::generateTag('tr', ['class' => [OverviewTable::$class, 'filter']]);
      foreach ($this->slatJoints as $factory)
      {
        $ret .= $factory->getHtmlColumnFilter();
      }
      $ret .= '<th class="error"></th>';
      $ret .= '</tr>';
    }

    return $ret;
  }