(define my-abs
  (lambda (n)
    (if (positive? n)
	n
	(- n))))
(define times3
  (lambda (n)
    (* n 3)))
(define times6
  (lambda (n)
    (+ (times3 n) (times3 n))))
(define fact
  (lambda (n)
    (if (zero? n)
	1
	(* n (fact (- n 1))))))
(define nth-element
  (lambda (ls n)
    (cond [(null? ls) "error"]
	  [(zero? n) (car ls)]
	  [else (nth-element (cdr ls) (- n 1))])))(nth-element