private function addValuesOptions($dom, $elem)
    {
        if (!empty($this->valuesOptions)) {
            foreach($this->valuesOptions as $opt) {
                $valuesElem = $dom->createElement('values-option');
                $valuesElem->nodeValue = $opt;
                $elem->appendChild($valuesElem);
            }
        }
        return $elem;
    }