anchor.diff:  							    user-pixmap-y1
   							    user-pixmap-x2
   							    user-pixmap-y2)))
  -			      (set-native sheet-native pixmap-region  sheet)
  +			      (set-native sheet-native pixmap-region  msheet)
   			      ;(break)
   			      (with-drawing-options
   				  (medium :ink (medium-background medium))
  
positive.code: 	       (pixmap-y2 (ceiling sheet-y2))
  	       (pixmap-width (- pixmap-x2 pixmap-x1))
  	       (pixmap-height (- pixmap-y2 pixmap-y1))
  	       (current-sheet-region (sheet-region sheet))
  	       (sheet-native (compose-transformation-with-translation
  			      sheet-transform
  			      (- pixmap-x1)
  			      (- pixmap-y1)))
  	       (pixmap (allocate-pixmap sheet pixmap-width pixmap-height))
  	       )
  	  (unless pixmap
  	    (error "Couldn't allocate pixmap")) 
  
negative.code:              (replay-output-record ,record ,sheet))))))
  
  (defun invoke-with-double-buffering (sheet continuation x1 y1 x2 y2)
    (let* ((msheet (sheet-mirrored-ancestor sheet))
  	 (medium (sheet-medium sheet))
  	 (sheet-transform (sheet-native-transformation sheet))
  	 (medium-transform (medium-transformation (sheet-medium sheet)))
  	 (world-transform (compose-transformations
  			   sheet-transform
  			   medium-transform)))
      (multiple-value-bind (sheet-x1 sheet-y1)
  	(transform-position world-transform x1 y1)
        (multiple-value-bind (sheet-x2 sheet-y2)
  