public function optgroup(Optgroup $optgroup)
    {
        $innerHtml = '';
        foreach ($optgroup->getOptions() as $option) {
            $innerHtml .= $this->render($option) . PHP_EOL;
        }

        $tag = new Tag('optgroup', $innerHtml);
        $tag->setAttribute('label', $optgroup->getLabel());

        return $this->parseTag($tag, $optgroup);
    }