'Append message to the list of un-ACKed messages.
Add a message, referenced by the delivery_tag, for ACKing,
rejecting, or getting later. Messages are saved into an
:class:`collections.OrderedDict` by delivery_tag.
:param message: A received message that has not yet been ACKed.
:type message: qpid.messaging.Message
:param delivery_tag: A UUID to refer to this message by
upon receipt.
:type delivery_tag: uuid.UUID'

def append(self, message, delivery_tag):

    self._not_yet_acked[delivery_tag] = message
