SICP 問題1.10

(define (A x y)
  (cond ((= y 0) 0)
        ((= x 0) (* 2 y))
        ((= y 1) 2)
        (else (A (- x 1)
                 (A x (- y 1))))))

(A 1 10)
;; gosh> 1024

(A 2 4)
;; gosh> 65536

(A 3 3)
;; gosh> 65536

(define (f n) (A 0 n))
;; => f(n) = 2n

(define (g n) (A 1 n))
;; => g(n) = 2^n

(define (h n) (A 2 n))
;; => h(1) = 2
;; => h(2) = 2^2
;; => h(3) = 2^(2^2)
;; => h(4) = 2^(2^(2^2))
;; 2の2乗をn回繰り返したもの？