anchor.diff:  	 (old-children (if (slot-boundp record 'old-children)
   			   (old-children record)
   			   nil)))
  -    (unless (or (region-intersects-region-p visible-region record)
  +    (unless (or (null visible-region)
  +		(region-intersects-region-p visible-region record)
   		(and old-children
   		     (region-intersects-region-p visible-region old-children)))
         (return-from compute-difference-set (values nil nil nil nil nil)))
  
positive.code: 
  (defmethod print-object ((obj standard-updating-output-record) stream)
    (print-unreadable-object (obj stream :type t :identity t)
      (when (slot-boundp obj 'x1)
        (with-slots (x1 y1 x2 y2) obj
  	(format stream "X ~S:~S Y ~S:~S " x1 x2 y1 y2))
        (format stream "~S" (output-record-dirty obj)))
      (when (slot-boundp obj 'unique-id)
        (let ((*print-length* 10)
  	    (*print-level* 3))
  
negative.code:       rect
      (make-bounding-rectangle min-x min-y max-x max-y)))
  
  ;;; work in progress
  (defvar *existing-output-records* nil)
  
  (defmethod compute-difference-set ((record standard-updating-output-record)
  