<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 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