docs: def demux_adaptor(stream_id, data):
      """
      Utility to demultiplex stdout and stderr when reading frames from the
      socket.
      """
      if stream_id == STDOUT:
          return (data, None)
      elif stream_id == STDERR:
          return (None, data)
      else:
          raise ValueError('{0} is not a valid stream'.format(stream_id))