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

(display (ack 3 5)) (newline)
(display (ack 3 6)) (newline)
(display (ack 3 7)) (newline)