functor
  (Tree : Graph.Graphviz.GraphWithDotAttrs) (TreeManipulation : sig
                                                                  val is_ghost_node :
                                                                    Tree.V.t ->
                                                                    bool
                                                                end) ->
  sig
    val from_tree :
      [> `widget ] Gtk.obj ->
      Tree.t -> Tree.V.t -> XDot.Make(Tree).graph_layout
  end