Ex-2.37

(define (matrix-*-vector m v)
  (map
   (lambda (w)
     (dot-product v w)) m))
(define (transpose mat)
  (accumulate-n
   (lambda (x y) (cons x y))
   '() mat))
(define (matrix-*-matrix m n)
  (let ((cols (transpose n)))
    (map
     (lambda (v)
       (map (lambda (w) (dot-product v w)) cols))
     m)))