public function city($province_id = NULL, $city_id = NULL) {
        $params = (is_null($province_id)) ? array() : array('province' => $province_id);
        if (!is_null($city_id)) {
            $params['id'] = $city_id;
        }
        $rest_client = new RESTClient($this->api_key, 'city', $this->account_type);
        return $rest_client->get($params);
    }