code: def _get_broadcasts(tables):
      
      tables = set(tables)
      casts = tz.keyfilter(
          lambda x: x[0] in tables and x[1] in tables, _BROADCASTS)
      if tables - set(tz.concat(casts.keys())):
          raise ValueError('Not enough links to merge all tables.')
      return casts
docs: Get the broadcasts associated with a set of tables.
  
      Parameters
      ----------
      tables : sequence of str
          Table names for which broadcasts have been registered.
  
      Returns
      -------
      casts : dict of `Broadcast`
          Keys are tuples of strings like (cast_name, onto_name).