anchor.diff:  
   (defclass rgb-image-design (design)
       ((medium :initform nil :initarg :medium)
  -     (image :initarg :image)
  +     (image :reader image
  +            :initarg :image)
        (medium-data :initform nil)))
   
   (defun make-rgb-image-design (image)
  
positive.code:       (setf medium current-medium)
        (setf medium-data nil))))
  
  
  ;;; Fetching protocol
  
  
negative.code: (defgeneric sheet-rgb-data (port sheet &key x y width height))
  
  
  (defmethod draw-design
      (medium (design rgb-image-design) &rest options
       &key x y &allow-other-keys)
    (unless (and x y)
      (setf (values x y) (stream-cursor-position medium)))
    (with-medium-options (medium options)
      (medium-draw-image-design* medium design x y)))
  