docs: def get_broadcast(cast_name, onto_name):
      """
      Get a single broadcast.
  
      Broadcasts are stored data about how to do a Pandas join.
      A Broadcast object is a namedtuple with these attributes:
  
          - cast: the name of the table being broadcast
          - onto: the name of the table onto which "cast" is broadcast
          - cast_on: The optional name of a column on which to join.
            None if the table index will be used instead.
          - onto_on: The optional name of a column on which to join.
            None if the table index will be used instead.
          - cast_index: True if the table index should be used for the join.
          - onto_index: True if the table index should be used for the join.
  
      Parameters
      ----------
      cast_name : str
          The name of the table being braodcast.
      onto_name : str
          The name of the table onto which `cast_name` is broadcast.
  
      Returns
      -------
      broadcast : Broadcast
  
      """
      if is_broadcast(cast_name, onto_name):
          return _BROADCASTS[(cast_name, onto_name)]
      else:
          raise KeyError(
              'no rule found for broadcasting {!r} onto {!r}'.format(
                  cast_name, onto_name))