> (trace fact)
(fact)
> (fact 5)
0: (fact 5)
1: (fact 4)
2: (fact 3)
3: (fact 2)
4: (fact 1)
5: (fact 0)
5: returned 1
4: returned 1
3: returned 2
2: returned 6
1: returned 24
0: returned 120
120