protected function processMessageCancel(Message $itipMessage, VCalendar $existingObject = null)
    {
        if (!$existingObject) {
            // The event didn't exist in the first place, so we're just
            // ignoring this message.
        } else {
            foreach ($existingObject->VEVENT as $vevent) {
                $vevent->STATUS = 'CANCELLED';
                $vevent->SEQUENCE = $itipMessage->sequence;
            }
        }

        return $existingObject;
    }