(set) x = 10
(set) y = 20
x + y = 30
x is 10 and y is 20
(call) add args=(2, 3)
(set) add.a = 2
(set) add.b = 3
(set) add.total = 5
(return) add args=(2, 3) -> 5
(change) x = 15
