anchor.diff:                           (mirror (sheet-direct-mirror sheet)))
                        (port-set-mirror-region port mirror MR)
                        (port-set-mirror-transformation port mirror MT)))
  -                 ;; update the native transformation if neccessary.
  +                 ;; update the native transformation if necessary.
                    (unless (and old-native-transformation
                                 (transformation-equal native-transformation
                                                       old-native-transformation))
  
positive.code:                (MT (make-translation-transformation x1 y1))
                 (MR (make-rectangle* 0 0 (round (- x2 x1)) (round (- y2 y1))))
                 (native-transformation
                  ;; NT = T o PNT o -MT
                  (compose-transformations
                   (invert-transformation MT)
                   (compose-transformations (sheet-native-transformation (sheet-parent sheet))
                                            (sheet-transformation sheet))))
                 (old-native-transformation
                  (%%sheet-native-transformation sheet)))
  
            (cond ((and (> (round (- x2 x1)) 0)
                        (> (round (- y2 y1)) 0))
  
negative.code:   - the parent's transformation changed
    - the parent's mirror region changed
  
  Also if the sheet's native transformation changes, the mirror's contents need
  to be redrawn, which is achieved by calling PORT-DIRTY-MIRROR-REGION. TODO is this true?
  
  Since changing the sheet's native transformation might thus be expensive,
  this function tries to minimize changes to it. (although it does not try
  