anchor.diff:  (defparameter slider-button-long-dim 30)
   (defparameter slider-button-short-dim 10)
   
  -(defclass slider-pane (slider-gadget activate/deactivate-repaint-mixin basic-pane)
  -  ((drag-callback  :initform nil
  -                   :initarg :drag-callback
  -                   :reader slider-drag-callback)
  -   (show-value-p   :type boolean
  -                   :initform nil
  -                   :initarg :show-value-p
  -                   :accessor gadget-show-value-p)
  -   (decimal-places :initform 0
  -                   :initarg :decimal-places
  -                   :reader slider-decimal-places)
  -   (number-of-quanta :initform nil
  -                     :initarg :number-of-quanta
  -                     :reader slider-number-of-quanta)))
  +(defclass slider-pane (slider
  +		       gadget-color-mixin
  +		       value-changed-repaint-mixin
  +		       activate/deactivate-repaint-mixin
  +		       basic-pane)
  +  ())
   
   (defmethod compose-space ((pane slider-pane) &key width height)
     (declare (ignore width height))
  
positive.code:                               :min-height major :height major)
      (make-space-requirement :min-width  major :width  major
                              :min-height minor :height minor))))
  
  
  
  (defmethod initialize-instance :before ((pane slider-pane) &rest rest)
    (declare (ignore rest))
    (setf (slot-value pane 'orientation) :vertical))
  
  (defmethod drag-callback ((pane slider-pane) client gadget-id value)
    (declare (ignore client gadget-id))
    (when (slider-drag-callback pane)
      (funcall (slider-drag-callback pane) pane value)))
  
  (defmethod handle-event ((pane slider-pane) (event pointer-enter-event))
    (with-slots (armed) pane
  
negative.code: 
  ;;; 30.4.5 The abstract slider Gadget
  
  (defclass slider (labelled-gadget-mixin
  		  value-gadget
  		  oriented-gadget-mixin
  		  range-gadget-mixin)
    ((drag-callback  :initform nil
                     :initarg :drag-callback
                     :reader slider-drag-callback)
     (show-value-p   :type boolean
                     :initform nil
                     :initarg :show-value-p
                     :accessor gadget-show-value-p)
     (decimal-places :initform 0
                     :initarg :decimal-places
                     :reader slider-decimal-places)
     (number-of-quanta :initform nil
                       :initarg :number-of-quanta
                       :reader slider-number-of-quanta))
    (:documentation "The value is a real number, and default value for orientation is :vertical,
  and must never be nil.")
    (:default-initargs :orientation :vertical))
  
  (defmethod drag-callback ((pane slider) client gadget-id value)
    (declare (ignore client gadget-id))
    (when (slider-drag-callback pane)
      (funcall (slider-drag-callback pane) pane value)))
  
  ;;; 30.4.6 The abstract radio-box and check-box Gadgets
  
  