(define (fact x)
  (if (= x 0) 1
      (* x (fact (- x 1)))))
;
(fact 3)
= (if (= 3 0) 1 (* 3 (fact (- 3 1))))
  (fact 2)
  = (if (= 2 0) 1 (* 2 (fact (- 2 1))))
    (fact 1)
    = (if (= 1 0) 1 (* 1 (fact (- 1 1))))
       (fact 0)
       = (if (= 0 0) 1 (* 0 (fact (- 0 1))))
       = 1
    = (* 1 1)
    = 1
  = (* 2 1)
  = 2
= (* 3 2)
= 6