anchor.diff:  
   (defun eye (n)
     "Identity matrix"
  -  (let ((m (make-array (list n n))))
  +  (let ((m (make-array (list n n) :initial-element 0)))
       (dotimes (i n)
         (setf (aref m i i) 1))
       m))
  
positive.code: (defmeth2 * ((a array) (b array))
    (destructuring-bind (m p) (array-dimensions a)
      (destructuring-bind (p n) (array-dimensions b)
        (let ((c (make-array (list m n))))
  	(dotimes (i m)
  	  (dotimes (j n)
  	    (setf (aref c i j) 
  
negative.code: (defun mapm (fn &rest ms)
    "Map a function across the elements of matrices of the same dimension"
    (destructuring-bind (r c) (array-dimensions (first ms))
      (let ((mout (make-array (list r c) :initial-element 0)))
        (dotimes (ri r)
  	(dotimes (ci c)
  	  (setf (aref mout ri ci) 
  