code: def _validate_first_message(cls, msg):
          
          data = cls._unpack_message(msg)
          logger.debug(data)
          if data != cls.RTM_HANDSHAKE:
              raise SlackApiError('Unexpected response: {!r}'.format(data))
          logger.info('Joined real-time messaging.')
docs: Check the first message matches the expected handshake.
  
          Note:
            The handshake is provided as :py:attr:`RTM_HANDSHAKE`.
  
          Arguments:
            msg (:py:class:`aiohttp.Message`): The message to validate.
  
          Raises:
            :py:class:`SlackApiError`: If the data doesn't match the
              expected handshake.