(define nth-element
  (lambda (ls n)
    (cond [(null? ls) "error"]
	  [(zero? n) (car ls)]
	  [else (nth-element (cdr ls) (- n 1))])))

(define nth-element
  (lambda (ls n)
    (if (null? ls)
	"error"
	(if (zero? n)
	    (car ls)
	    (nth-element (cdr ls) (- n 1))])))

(define fact
  (lambda (n)
    (cond [(zero? n) 1]
	  [(negative? n) "Error: negative input to fact"]
	  [else (* n (fact (- n 1)))])))

(define fact2
  (lambda (n)
    (if (or (negative? n)
	    (not (integer? n)))
	"error"
	(fact-acc n 1)))) ; acc stands for accumulator

(define fact-acc
  (lambda (n acc)
    (if (zero? n)
	acc
	(fact-acc (- n 1) (* n acc)))))
(fact2 4)