protected function unmarshallChildrenKnown(DOMElement $element, QtiComponentCollection $children)
    {
        if (($name = $this->getDOMElementAttributeAs($element, 'name')) !== null) {

            $object = new MathOperator($children, MathFunctions::getConstantByName($name));

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