'Retrieves a list of stored messages. Returns a tuple of the messages
and a flag indicating whether or not all the messages originally
intended to be stored in this storage were, in fact, stored and
retrieved; e.g., ``(messages, all_retrieved)``.
**This method must be implemented by a subclass.**
If it is possible to tell if the backend was not used (as opposed to
just containing no messages) then ``None`` should be returned in
place of ``messages``.'

def _get(self, *args, **kwargs):

    raise NotImplementedError()
