(defun mmul (A B)
  (let* ((m (car (array-dimensions A)))
         (n (cadr (array-dimensions A)))
         (l (cadr (array-dimensions B)))
         (C (make-array `(,m ,l) :initial-element 0)))
    (loop for i from 0 to (- m 1) do
              (loop for k from 0 to (- l 1) do
                    (setf (aref C i k)
                          (loop for j from 0 to (- n 1)
                                sum (* (aref A i j)
                                       (aref B j k))))))
    C))