def set_topic(self, new_topic):
        """
        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.
        """

        msg = aioxmpp.stanza.Message(
            type_=aioxmpp.structs.MessageType.GROUPCHAT,
            to=self._mucjid
        )
        msg.subject.update(new_topic)

        yield from self.service.client.send(msg)