public function getETag()
    {
        if (isset($this->cardData['etag'])) {
            return $this->cardData['etag'];
        } else {
            $data = $this->get();
            if (is_string($data)) {
                return '"'.md5($data).'"';
            } else {
                // We refuse to calculate the md5 if it's a stream.
                return null;
            }
        }
    }