public function resend($method = null)
    {
        if ($method) {
            if (!in_array($method, array('Email', 'Letter')))
                throw new InvalidFieldValueError("'method' must be either " .
                    "'Email' or 'Letter'");

            return $this->performEvent('resend', array('method' => $method));
        }

        return $this->performEvent('resend');
    }