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Does this make sense?
(define W1 (make-withdraw 100))
When make-withdraw is evaluated, E0 is created with Frame A having the
initial-mount binding. Next, as a result of the evaluation of the
anonymous function (generated by the set (Edit: let?) structure), Frame B is created
with the binding of balance (E1 is the pointer to this frame).
_______________________
global->| make-withdraw : * |
env. | W1 : * | |
-------|---^-----|---^-
| | | |
| | parameter: initial-mount
| | body: ((lambda (balance) ((...))) initial-mount)
| |
| _|___Frame_A__________
| | initial-mount : 100 |<- E0
| -^--------------------
| |
| _|__________Frame_B______
| | balance : initial-mount | <- E1
| -^-----------------------
| |
parameter: amount
body: (if (>= balance amount) ... )
(W1 50)
Set! will affect Frame B, initial-mount remains unchanged in Frame A.
_______________________
global->| make-withdraw : * |
env. | W1 : * | |
-------|---^-----|---^-
| | | |
| | parameter: initial-mount
| | body: ((lambda (balance) ((...))) initial-mount)
| |
| _|___Frame_A__________
| | initial-mount : 100 |<- E0
| -^--------------------
| |
| _|__________Frame_B___
| | balance : 50 | <- E1
| -^--------------------
| |
parameter: amount
body: (if (>= balance amount) ... )
It's interesting to see that the redundant lambda expression created a redundant frame. The W1 procedure will only interact with the second created frame that has the value of balance.