anchor.diff:  (defclass mirrored-pixmap (pixmap)
     ((port :initform nil :initarg :port :accessor port)
      (medium :initform nil :accessor pixmap-medium)
  -   (region :initform nil :accessor sheet-region)
  -   ))
  +   (region :initform nil :accessor sheet-region)))
   
  -; added this. CHECKME -- BTS
  +(defgeneric (setf %sheet-medium) (medium sheet))
  +
  +;;; added this. CHECKME -- BTS
   (defmethod (setf %sheet-medium) (value (pixmap mirrored-pixmap))
     (setf (slot-value pixmap 'medium) value))
   
  
positive.code: (defclass pixmap ()
    ((sheet :initarg :sheet :reader pixmap-sheet)
     (width :initarg :width :reader pixmap-width)
     (height :initarg :height :reader pixmap-height)
     ))
  
  (defgeneric pixmap-mirror (mirrored-pixmap))
  (defgeneric allocate-pixmap (sheet width height))
  
negative.code: (defmethod invalidate-cached-regions ((sheet mirrored-pixmap))
    (values))
  
  ;;;; BTS stopped adding. ^-- CHECKME
  
  (defmethod initialize-instance :after ((pixmap mirrored-pixmap) &rest args)
    (declare (ignore args))
  