public function getRegisteredUserFunctions($namespace = null)
    {
        $url = UrlHelper::buildUrl(Urls::URL_AQL_USER_FUNCTION, []);
        if (null !== $namespace) {
            $url = UrlHelper::appendParamsUrl($url, ['namespace' => $namespace]);
        }
        $response = $this->_connection->get($url);

        $data = $response->getJson();
        if (isset($data['result'])) {
            return $data['result'];
        }
        // backwards compatibility
        return $data;
    }