(defparameter m1 '((1 2 3) (4 5 6)))
(defparameter m2 '((1 0 0) (0 1 0) (0 0 1)))


(defun mul-matrix (a b)
  (mapcar (lambda (x)
            (apply #'mapcar (lambda (&rest y)
                              (apply #'+ (mapcar #'* x y)))
                   b))
          a))