The set of all subsets of a set: powerset(A), the P(A) of set theory.
Its members are sets, and a set is one of them exactly when it is a subset of
Argument, which is how membership is answered whether or not the
argument is finite: {1, 3, 7} in powerset(ZZ) is True. For a finite
argument it evaluates to the list of its 2^n subsets, the empty set and the
whole among them; powerset({}) is { {} }, one member, not none.
https://github.com/asc-community/AngouriMath/issues/1409