anchor.diff:        (gl:vertex (+ (gl-x w) (* slider-scale (gui-slider-value w)))
                    (- (gl-y w) (height w))))))
   
  -(defmethod gui-widget-mouse-down ((it gui-slider))
  +(defmethod gui-widget-mouse-down ((it gui-slider) (btn (eql :left-button)))
     (setf (gui-slider-sliding it) t))
   
  -(defmethod gui-widget-mouse-up ((it gui-slider))
  +(defmethod gui-widget-mouse-up ((it gui-slider) (btn (eql :left-button)))
     (setf (gui-slider-sliding it) nil))
   
   (defmethod gui-widget-mouse-move ((it gui-slider) dx dy)
  
positive.code:           (setf entered-widget widget))
          (gui-widget-mouse-move widget dx dy)))))
  
  (defclass gui-widget ()
    (;; relative coordinates for absolute position
     ;; may be overriden by layout if not :absolute
  
negative.code:     (setf (gui-widgets it) (rotate-list-right (gui-widgets it) 1))
      (focus (gui-focus it))))
  
  (defun gui-widget-at (gui x y)
    (loop for w in (gui-widgets gui)
       for found = (gui-widget-child-at w x y)
  