Exercise 2.41.  Write a procedure to find all ordered triples of 
distinct positive integers i, j, and k less than or equal to a given 
integer n that sum to a given integer s.

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

(define (sums-to s)
  (lambda (triple)
    (= s (fold-left + 0 triple))))

(define (triples-summing-to n s)
  (filter (sums-to s) (unique-triples n)))