code: def set_topic(self, new_topic):
          
  
          msg = aioxmpp.stanza.Message(
              type_=aioxmpp.structs.MessageType.GROUPCHAT,
              to=self._mucjid
          )
          msg.subject.update(new_topic)
  
          yield from self.service.client.send(msg)
docs: Change the (possibly publicly) visible topic of the conversation.
  
          :param new_topic: The new topic for the conversation.
          :type new_topic: :class:`str`
  
          Request to set the subject to `new_topic`. `new_topic` must be a
          mapping which maps :class:`~.structs.LanguageTag` tags to strings;
          :data:`None` is a valid key.