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: (defclass generic-tree-pane (tree-pane meta-tree-pane
                                         standard-sheet-input-mixin ;; Hmm..
                                         value-changed-repaint-mixin
                                         mouse-wheel-scroll-mixin
                                         sheet-padding-mixin)
    ((highlight-ink :initform +royalblue4+
                    :initarg :highlight-ink
                    :reader tree-pane-highlight-ink)
  