;; Transpose a mxn matrix A to a nxm matrix B=A'.
(defun mtp (A)
  (let* ((m (array-dimension A 0))
         (n (array-dimension A 1))
         (B (make-array `(,n ,m) :initial-element 0)))
    (loop for i from 0 below m do
          (loop for j from 0 below n do
                (setf (aref B j i)
                      (aref A i j))))
    B))