anchor.diff:        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)
  
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: 	 (old-children (if (slot-boundp record 'old-children)
  			   (old-children record)
  			   nil)))
      (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)))
  