<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def reject(self):
 """Reject this message. The message will be discarded by the server. :raises MessageStateError: If the message has already been acknowledged/requeued/rejected. """

        if self.acknowledged:
            raise self.MessageStateError(
                "Message already acknowledged with state: %s" % self._state)
        self.backend.reject(self.delivery_tag)
        self._state = "REJECTED"