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).

def _get_broadcasts(tables):
    """
    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).

    """
    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