anchor.diff:  (defclass pixmap ()
     ((sheet :initarg :sheet :reader pixmap-sheet)
      (width :initarg :width :reader pixmap-width)
  -   (height :initarg :height :reader pixmap-height)
  -   ))
  +   (height :initarg :height :reader pixmap-height)))
   
   (defgeneric pixmap-mirror (mirrored-pixmap))
   (defgeneric allocate-pixmap (sheet width height))
  
positive.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))
  
negative.code: (defclass mirrored-pixmap (pixmap)
    ((port :initform nil :initarg :port :accessor port)
     (medium :initform nil :accessor pixmap-medium)
     (region :initform nil :accessor sheet-region)))
  
  (defgeneric (setf %sheet-medium) (medium sheet))
  
  ;;; added this. CHECKME -- BTS
  (defmethod (setf %sheet-medium) (value (pixmap mirrored-pixmap))
    (setf (slot-value pixmap 'medium) value))
  
  