public function unmarshall(DOMElement $element)
    {
        if (($identifier = $this->getDOMElementAttributeAs($element, 'identifier')) !== null) {

            if (($href = $this->getDOMElementAttributeAs($element, 'href')) !== null) {
                return new RubricBlockRef($identifier, $href);
            } else {
                $msg = "The mandatory 'href' attribute is missing from element 'rubricBlockRef'.";
                throw new UnmarshallingException($msg, $element);
            }
        } else {
            $msg = "The mandatory 'identifier' attribute is missing from element 'rubricBlockRef'.";
            throw new UnmarshallingException($msg, $element);
        }
    }