docs: def broadcast(cast, onto, cast_on=None, onto_on=None,
                cast_index=False, onto_index=False):
      """
      Register a rule for merging two tables by broadcasting one onto
      the other.
  
      Parameters
      ----------
      cast, onto : str
          Names of registered tables.
      cast_on, onto_on : str, optional
          Column names used for merge, equivalent of ``left_on``/``right_on``
          parameters of pandas.merge.
      cast_index, onto_index : bool, optional
          Whether to use table indexes for merge. Equivalent of
          ``left_index``/``right_index`` parameters of pandas.merge.
  
      """
      logger.debug(
          'registering broadcast of table {!r} onto {!r}'.format(cast, onto))
      _BROADCASTS[(cast, onto)] = \
          Broadcast(cast, onto, cast_on, onto_on, cast_index, onto_index)