(define (fact x)
  (if (= x 0) 1
      (* x (fact (- x 1)))))

(define (fact x)
  (define (fact-tail x accum)
    (if (= x 0) accum
        (fact-tail (- x 1) (* x accum))))
  (fact-tail x 1))


(fact 3)
		    ;cont   ;acc
(fact-tail (- 3 1) (* 3 1))
		      2       3
	(fact-tail (- 2 1) (* 2 3))
				1       6
		(fact-tail (- 1 1) (* 1 6))
					0       6
			(fact-tail (- 0 1) (* 0 6))
                          -1        base-case: return 6