public function prepare(array $params)
    {
        $params['wxappid'] = $this->merchant->app_id;

        // XXX: PLEASE DON'T CHANGE THE FOLLOWING LINES.
        $params['auth_mchid'] = '1000052601';
        $params['auth_appid'] = 'wxbf42bd79c4391863';

        $params['amt_type'] = 'ALL_RAND';

        return $this->request(self::API_PREPARE, $params);
    }