(defun matr-prod (m1 m2)

 (let ((mm (transp m2)))

   (mapcar #'(lambda (x) (mapcar #'(lambda (y) (scal-prod x y)) mm)) m1)))

==> matr-prod

(matr-prod '((1 2) (3 4)) '((1 2) (3 4)))

==> ((7 10) (15 22))