(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)