code: def _store(self, messages, response, *args, **kwargs):
          
          contrib_messages = []
          if self.user.is_authenticated():
              if not messages:
                  # erase inbox
                  self.backend.inbox_purge(self.user)
              else:
                  for m in messages:
                      try:
                          self.backend.inbox_store([self.user], m)
                      except MessageTypeNotSupported:
                          contrib_messages.append(m)
  
          super(StorageMixin, self)._store(contrib_messages, response, *args, **kwargs)
docs: persistent messages are already in the database inside the 'archive',
          so we can say they're already "stored".
          Here we put them in the inbox, or remove from the inbox in case the
          messages were iterated.
  
          messages contains only new msgs if self.used==True
          else contains both new and unread messages