public function getHtmlHeadEntrySet()
    {
        $set = parent::getHtmlHeadEntrySet();

        foreach ($this->columns as $column) {
            $set->addEntrySet($column->getHtmlHeadEntrySet());
        }

        foreach ($this->spanning_columns as $column) {
            $set->addEntrySet($column->getHtmlHeadEntrySet());
        }

        foreach ($this->extra_rows as $row) {
            $set->addEntrySet($row->getHtmlHeadEntrySet());
        }

        foreach ($this->groups as $group) {
            $set->addEntrySet($group->getHtmlHeadEntrySet());
        }

        return $set;
    }