element ( element ), , "", set, assignment, test, size
set ( set ) { element, ... }, var, assignment, set operation
(tests, result is 0 if false, 1 if true)
subset S1 <= S2
proper subset S1 < S2
equal S1 = S2
not equal S1 != S2
is in set e [ S
is not in set e ![ S
(set operations, result is set)
union S1 + S2
intersection S1 ^ S2
difference S1 - S2
symmetric diff S1 / S2
powerset @ S
assignment var : element
(operations, result is number)
size |S|