walker...to the next step

(define (triple-sum n s)
  (filter
    (lambda (tri)
      (if (= s
             (+ (car tri)
                (cadr tri)
                (caddr tri)))
        #t
        #f))
    (flatmap (lambda (i)
               (flatmap (lambda (j)
                          (map (lambda (k)
                                 (list i j k))
                               (enumerate-interval 1 n)))
                        (enumerate-interval 1 n)))
             (enumerate-interval 1 n))))