code: def add_event(self, event_):
          
          conv_event = self._wrap_event(event_)
          if conv_event.id_ not in self._events_dict:
              self._events.append(conv_event)
              self._events_dict[conv_event.id_] = conv_event
          else:
              # If this happens, there's probably a bug.
              logger.info('Conversation %s ignoring duplicate event %s',
                          self.id_, conv_event.id_)
              return None
          return conv_event
docs: Add an event to the conversation.
  
          This method is used by :class:`.ConversationList` to maintain this
          instance.
  
          Args:
              event_: ``Event`` message.
  
          Returns:
              :class:`.ConversationEvent` representing the event.