docs: def powerset(iterable) -> FrozenSet:
      "powerset([1,2,3]) --> () (1,) (2,) (3,) (1,2) (1,3) (2,3) (1,2,3)"
      combs = _powerset(iterable)
      res = frozenset(frozenset(x) for x in combs)
      # res = map(frozenset, combs)
      return res