(define (assert a b)
  (cond ((equal? a b) (display "[ok]"))
    (else
      (display "[error] ")
      (display a)
      (display " != ")
      (display b)))
  (newline))

(define (gcd a b)
  (if (= b 0) (abs a) (gcd b (remainder a b))))

;(assert (gcd 93 15) 3)