The statement that every member of Sub is a member of
Super: A subset B, or A ⊆ B. The relation between sets
that in is between an element and a set, and a different one: {1, 2} is a subset of {1, 2, 3} and not an element of it. Decided from the shapes
of the two sets, by the set algebra, member by member for a finite set, and
through forall x in A : x in B where a bound reads as a comparison; two
sets are equal exactly when each is a subset of the other, which is how = between sets is decided. A statement about sets: NaN where a side is a
number or a truth value.
https://github.com/asc-community/AngouriMath/issues/1409