anchor.diff:  (assert (subtypep 'path 'region))
   (assert (subtypep 'path 'bounding-rectangle))
   
  -
  -
   ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
   ;;;
   ;;; area
  
positive.code:   (assert (or (typep d 'standard-region-difference)
  	      (pointp d)))
    (assert (member (length regions) '(1 2)))
    (assert (member p1 regions :test #'region-equal)))
  
negative.code: 
  ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
  ;;;
  ;;; coordinate
  
  (assert (or (and (subtypep 'coordinate t)
  		 (subtypep t 'coordinate))
  