SICP例題2.36

せっかくだから解いておく。

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

(define nil '())

(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)))))