public function cards($options = array())
    {
        if (is_array($options) && ! empty($options)) {
            $cards = parent::execute(self::getUrl($this['id']) . '/cards?' . http_build_query($options), parent::REQUEST_GET, parent::getResourceKey());
        } else {
            $cards = $this['cards'];
        }

        return new OmiseCardList($cards, $this['id'], $this->_publickey, $this->_secretkey);
    }