연습문제 2.37

(define (dot-product v w)
  (accumulate + 0 (map * v w)))

(define (matrix-*-vector m v)
  (map (lambda (x) (dot-product v x)) m))


(define (transpose mat)
  (accumulate-n cons nil mat))

(define (matrix-*-matrix m n)
  (let ((cols (transpose n)))
    (map (lambda (row) (matrix-*-vector cols row)) m)))