docs: def _send_rtm_message(self, channel_id, text):
          """Send a Slack message to a channel over RTM.
  
          :param channel_id: a slack channel id.
          :param text: a slack message. Serverside formatting is done
            in a similar way to normal user message; see
            `Slack's docs <https://api.slack.com/docs/formatting>`__.
          """
  
          message = {
              'id': self._current_message_id,
              'type': 'message',
              'channel': channel_id,
              'text': text,
          }
          self.ws.send(json.dumps(message))
          self._current_message_id += 1