def to_binary_string(self):
        """Pack the feedback to binary form and return it as string."""
        timestamp = datetime_to_timestamp(self.when)
        token = binascii.unhexlify(self.token)
        return struct.pack(self.FORMAT_PREFIX + '{0}s'.format(len(token)),
                           timestamp, len(token), token)
Pack the feedback to binary form and return it as string.