Consider the computation of (fact 3). A new stack frame is created in which n = 3:
| n = 3 |
Now we can execute the code of fact. We test (if (<= n 0) ...) and, since n = 3, evaluate (* n (fact (- n 1))). n evaluates to 3, and then we evaluate (fact (- n 1)) which is (fact 2). This creates a new stack frame in which n = 2:
| n = 2 |
| n = 3 |
Note that the older binding, n = 3, has not gone away, but is now shadowed by a new binding n = 2.
Now we test (if (<= n 0) ...) and call (fact 1):
| n = 1 |
| n = 2 |
| n = 3 |
and then we call (fact 0):
| n = 0 |
| n = 1 |
| n = 2 |
| n = 3 |