SICP 問題 2.40

(define (unique-pairs n)
  (flatmap
   (lambda (i)
     (map (lambda (j) (list i j))
          (enumerate-interval 1 (- i 1))))
   (enumerate-interval 1 n)))

(define (prime-sum-pairs n)
  (map make-pair-sum
       (filter prime-sum?
               (unique-pairs n))))