;; SICP
;; Exercise 2.40.
;;
;; Define a procedure unique-pairs that, given an integer n, generates the sequence of pairs
;; (i, j ) with 1 ≤ j ≤ i ≤ n. Use unique-pairs to simplify the definition of prime-sum-pairs given above.

(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))))