protected function getIndexKey($element)
    {
        $key = $element->getFullyQualifiedStructuralElementName();

        // properties should have an additional $ before the property name
        if ($element instanceof PropertyInterface) {
            list($fqcn, $propertyName) = explode('::', $key);
            $key = $fqcn . '::$' . $propertyName;
        }

        return $key;
    }