(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))))

(define (average a b) (/ (+ a b) 2.0))
(define (id n) n)
(define (inc n) (+ n 1))
(define nil '())