anchor.diff:    ```"
     (let ((displacement-step
             (or displacement-step
  -              (reduce #'* (alexandria:ensure-list dimensions))))
  +              (mat-size-from-dimensions (alexandria:ensure-list dimensions))))
           (displacement (mat-displacement mat))
           (size (mat-size mat)))
       (with-shape-and-displacement (mat)
  
positive.code: 
  (defmethod make-facet* ((mat mat) (facet-name (eql 'cuda-array)))
    (let ((array (alloc-cuda-array (* (mat-max-size mat)
                                      (cffi:foreign-type-size (mat-ctype mat))))))
      (when (and (mat-initial-element mat)
                 (endp (views mat)))
        (cuda-fill!-2 (mat-ctype mat) (mat-initial-element mat)
  
negative.code:    different representations of the same data. See @MAT-BASICS for a
     tuturialish treatment."))
  
  ;;; Faster version of CFFI:FOREIGN-TYPE-SIZE.
  (declaim (inline ctype-size))
  (defun ctype-size (ctype)
    (if (eq ctype :float)
        4
        8))
  
  (defmethod initialize-instance :after ((mat mat) &key initial-contents
                                         &allow-other-keys)
    (unless (listp (mat-dimensions mat))
      (setf (slot-value mat 'dimensions)
            (list (mat-dimensions mat))))
    (setf (slot-value mat 'size) (mat-size-from-dimensions (mat-dimensions mat)))
    (unless (slot-boundp mat 'max-size)
      (setf (slot-value mat 'max-size)
            (+ (mat-displacement mat) (mat-size mat))))
    (assert (<= (+ (mat-displacement mat) (mat-size mat)) (mat-max-size mat)))
    (setf (slot-value mat 'n-bytes)
          (* (mat-max-size mat) (ctype-size (mat-ctype mat))))
    (when (mat-initial-element mat)
      (setf (slot-value mat 'initial-element)
            (coerce-to-ctype (mat-initial-element mat)
                             :ctype (mat-ctype mat))))
    (when initial-contents
      (replace! mat initial-contents)))
  
  ;;; Optimized version of (REDUCE #'* DIMENSIONS).
  (defun mat-size-from-dimensions (dimensions)
    (let ((product 1))
      (declare (type index product)
               (optimize speed))
      (dolist (dimension dimensions)
        (declare (type index dimension))
        (setq product (the! index (* product dimension))))
      product))
  
  (defun mat-dimension (mat axis-number)
    "Return the dimension along AXIS-NUMBER. Similar to
  