anchor.diff:    (paint-generic-tree-pane-node
      (tree-pane-model pane)
      pane
  -   (sheet-region pane)
  +   (sheet-padding-region pane)
      0
      (indentation pane)))
   
  
positive.code:                  :documentation "Last action performed on items in the pane, either
  :select, :deselect, or NIL if none has been performed yet."))
    (:default-initargs :text-style (make-text-style :sans-serif :roman :normal)
      :background +white+ :foreground +black+))
  
  (defmethod initialize-instance :after ((gadget meta-tree-pane) &rest rest)
    (declare (ignorable rest))
  
negative.code: 
            ;; Draw the node's children y the node is opened
  
            (let ((next-region (make-bounding-rectangle x0
                                                        (+ y0 item-height)
                                                        x1
                                                        y1)))
              (when (member value (opened-nodes pane) :test (tree-pane-test pane))
                (loop
                   with children-indentation = (+ current-indentation indentation)
  