(defmethod mult ((m1 matrix-r) (m2 matrix-r))
  (if (not (= (number-of-cols m1) (number-of-rows m2)))
      (error "Can't perform matrix multiplication")
      (progn
    (let* ((elements-m1 (elements m1)) (elements-m2 (elements m2))
           (n (number-of-cols m1))
           (n-rows (number-of-rows m1))
           (n-cols (number-of-cols m2))
           (m (make-instance 'matrix-r :number-of-rows n-rows :number-of-cols n-cols))
           (elements-m (elements m)))
      (declare
       (type (simple-array double-float) elements-m1 elements-m2 elements-m)
       (type fixnum n-rows n-cols n)
       (optimize (debug 0) (safety 0) (speed 3)))
      (loop for i fixnum from 0 below n-rows do
           (loop for j fixnum from 0 below n-cols do
            (setf (aref elements-m (+ j (* i n-cols))) (loop for k fixnum from 0 below n
                        sum (* (aref elements-m1 (+ k (* i n))) (aref elements-m2 (+ j (* k n))))))))
      m))))