定番の例はたらいまわし関数だよな。

(define (gtarai x y z)
(gtarai
(gtarai (- x 1) y z)
(gtarai (- y 1) z x)
(gtarai (- z 1) x y)
))

(define (tarai x y z)
(if (<= (force x) (force y))
(force y)
(tarai (delay (tarai (- (force x) 1) y z))
(delay (tarai (- (force y) 1) z x))
(delay (tarai (- (force z) 1) x y)))))