anchor.diff:  (define-abstract-pane-mapping 'option-pane 'generic-option-pane)
   
   (defclass generic-option-pane (option-pane
  -                               meta-list-pane
                                  value-changed-repaint-mixin
                                  3d-border-mixin
                                  activate/deactivate-repaint-mixin
  
positive.code: 
  ;;; 30.4.7 The abstract list-pane and option-pane Gadgets
  
  (defclass list-pane (value-gadget)
    ()
    (:documentation
     "The instantiable class that implements an abstract list pane, that is, a gadget
      whose semantics are similar to a radio box or check box, but whose visual
      appearance is a list of buttons.")
    (:default-initargs :value nil))
  
  (defclass option-pane (value-gadget)
    ()
    (:documentation
     "The instantiable class that implements an abstract option pane, that is, a
  
negative.code:                  :documentation "Number of items")
     (items-origin :initform 0 :accessor items-origin
       :documentation "Index of the first item to be rendered. This changes in
  response to scroll wheel events."))
    (:default-initargs :background +white+ :foreground +black+))
  
  (defmethod initialize-instance :after ((gadget meta-list-pane) &rest rest)
  