SICP 問題 2.36

;; accumulate
(define (accumulate op initial sequence)
  (if (null? sequence)
      initial
      (op (car sequence)
          (accumulate op initial (cdr sequence)))))

;; accumulate-n
(define (accumulate-n op init seqs)
  (if (null? (car seqs))
             nil
             (cons (accumulate op init (map car seqs))
                   (accumulate-n op init (map cdr seqs)))))

gosh> (accumulate-n + 0 '((1 2 3) (4 5 6) (7 8 9) (10 11 12)))
(22 26 30)