; ackermann's function

;  (define x (runtime)) (ack 3 7) (- (runtime) x)

(define (ack m n)
      (cond ((= m 0) (+ n 1))
            ((= n 0) (ack (- m 1) 1))
            (else (ack (- m 1) (ack m (- n 1))))
      )
   )