difference (fromList [(5, "a"), (3, "b")]) (fromList [(5, "A"), (7, "C")]) == singleton 3 "b"
difference (fromList [(5, "a"), (3, "b")]) (fromList [(5, "A"), (7, "C")]) == singleton 3 "b"
difference (fromList [5, 3]) (fromList [5, 7]) == singleton 3
>>> HashSet.difference (HashSet.fromList [1,2,3]) (HashSet.fromList [2,3,4]) fromList [1]
difference (fromList [(5, "a"), (3, "b")]) (fromList [(5, "A"), (7, "C")]) == singleton 3 "b"