Some entries in Table()._entries are tuples of tuples of actions. They should just be tuples of actions.
See print(Table()._entries):
{(('delta',),): (set(), False), (('b', 'delta'),): (set(), False), (): ({'delta'}, True), ('a',): ({'x', 'delta'}, True), (('a', 'x'),): (set(), False), ('b',): ({'delta'}, True), (('a', 'delta'),): (set(), False)}