/**
  * Refund a payment.
  * Available $params are:
  * - amount: The amount to refund. The amount must not exceed the maximum
  *           amount of Payment. Can be used for partial refunds. If ommited
  *           then a full refund will be created. [Optional]
  * - description: A description for this refund max 255 chars. [Optional]
  *
  * @param string|stdClass $token A valid payment token returned from a
  *                               successful payment creation.
  * @param array $params
  */
 public static function refund($token, array $params = array())
 {
     $params['token_id'] = $token;
     return parent::invoke(__FUNCTION__, static::RESOURCE_NAME, $params);
 }