code: def add(self, event, subscriber, append=True):
          
          subs = self._subscribers
          if event not in subs:
              subs[event] = deque([subscriber])
          else:
              sq = subs[event]
              if append:
                  sq.append(subscriber)
              else:
                  sq.appendleft(subscriber)
docs: Add a subscriber for an event.
  
          :param event: The name of an event.
          :param subscriber: The subscriber to be added (and called when the
                             event is published).
          :param append: Whether to append or prepend the subscriber to an
                         existing subscriber list for the event.