Each time the function fact is entered, a new binding of n to its current value is created. These bindings are held on a stack, so that the new binding does not replace the old value of n, but shadows it. When the function fact exits, the binding of n is removed from the stack, leaving the previous value.
The special function trace is very useful for seeing the arguments and results of recursive calls:
: (trace fact) fact
: (fact 3) Entry (fact 3) |Entry (fact 2) | Entry (fact 1) | Entry (fact 0) | ==> 1 | ==> 1 |==> 2 ==> 6 6
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