code: def _on_watermark_notification(self, notif):
          
          # Update the conversation:
          if self.get_user(notif.user_id).is_self:
              logger.info('latest_read_timestamp for {} updated to {}'
                          .format(self.id_, notif.read_timestamp))
              self_conversation_state = (
                  self._conversation.self_conversation_state
              )
              self_conversation_state.self_read_state.latest_read_timestamp = (
                  parsers.to_timestamp(notif.read_timestamp)
              )
          # Update the participants' watermarks:
          previous_timestamp = self._watermarks.get(
              notif.user_id,
              datetime.datetime.min.replace(tzinfo=datetime.timezone.utc)
          )
          if notif.read_timestamp > previous_timestamp:
              logger.info(('latest_read_timestamp for conv {} participant {}' +
                           ' updated to {}').format(self.id_,
                                                    notif.user_id.chat_id,
                                                    notif.read_timestamp))
              self._watermarks[notif.user_id] = notif.read_timestamp
docs: Handle a watermark notification.