anchor.diff:  (defmethod map-sheet-position-to-child ((sheet sheet-parent-mixin) x y)
     (untransform-position (sheet-transformation sheet) x y))
   
  -(defmethod map-sheet-rectangle*-to-parent ((sheet sheet-parent-mixin) x1 y1 x2 y2)
  +(defmethod map-sheet-rectangle*-to-parent
  +    ((sheet sheet-parent-mixin) x1 y1 x2 y2)
     (transform-rectangle* (sheet-transformation sheet) x1 y1 x2 y2))
   
  -(defmethod map-sheet-rectangle*-to-child ((sheet sheet-parent-mixin) x1 y1 x2 y2)
  +(defmethod map-sheet-rectangle*-to-child
  +    ((sheet sheet-parent-mixin) x1 y1 x2 y2)
     (untransform-rectangle* (sheet-transformation sheet) x1 y1 x2 y2))
   
   ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
  
positive.code: 	      :initarg :enabled-p
                :initform t
                :accessor sheet-enabled-p)))
  ; Native region is volatile, and is only computed at the first request when it's equal to nil.
  ; Invalidate-cached-region method sets the native-region to nil.
  
  (defmethod sheet-parent ((sheet basic-sheet))
    nil)
  
negative.code: (defclass sheet-translation-transformation-mixin (sheet-transformation-mixin)
    ())
  
  (defmethod (setf sheet-transformation) :before
      ((transformation transformation)
       (sheet sheet-translation-transformation-mixin))
    (if (not (translation-transformation-p transformation))
        (error "Attempting to set the SHEET-TRANSFORMATION of a SHEET-TRANSLATION-TRANSFORMATION-MIXIN to a non translation transformation")))
  
  