anchor.diff:  
   (defmethod (setf medium-clipping-region) :after (clipping-region (medium medium))
     (declare (ignore clipping-region))
  -  (medium-invalidate-cached-device-region (medium-sheet medium)))
  +  (when (medium-sheet medium)
  +    (medium-invalidate-cached-device-region (medium-sheet medium))))
   
   (defmethod medium-device-region :before ((medium medium))
     (with-slots (device-region) medium
  
positive.code: 
  (defmethod (setf medium-transformation) :after (transformation (medium medium))
    (declare (ignore transformation))
    (medium-invalidate-cached-device-transformation (medium-sheet medium)))
  
  (defmethod medium-device-transformation :before ((medium medium))
    (with-slots (device-transformation) medium
  
negative.code:                        :initform (make-text-style :fix :roman :normal)
  		       :accessor medium-default-text-style)
     (sheet :initarg :sheet
  	  :initform nil ; this means that medium is not linked to a sheet
  	  :accessor medium-sheet)
     ))
  
  