protected function unmarshall(DOMElement $element)
    {
        if (($identifier = $this->getDOMElementAttributeAs($element, 'identifier')) !== null) {
            $object = new Variable($identifier);

            if (($weightIdentifier = $this->getDOMElementAttributeAs($element, 'weightIdentifier')) !== null) {
                $object->setWeightIdentifier($weightIdentifier);
            }

            return $object;
        } else {
            $msg = "The mandatory attribute 'identifier' is missing from element '" . $element->localName . "'.";
            throw new UnmarshallingException($msg, $element);
        }
    }