anchor.diff:  
   ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
   ;;;
  -;;; coordiante
  +;;; coordinate
   
   (assert (or (and (subtypep 'coordinate t)
   		 (subtypep t 'coordinate))
  
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: (assert (subtypep 'path 'region))
  (assert (subtypep 'path 'bounding-rectangle))
  
  ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
  ;;;
  ;;; area
  