# Module type DGraphSubTree.S

    module type S = sig .. end

    module Tree: DGraphSubTree.Tree  with type E.label = unit

    type t

    val get_structure : t -> Tree.t

    val get_root : t -> Tree.V.t

    val get_tree_vertices : Tree.V.label ->
           t -> Tree.V.t list

    val is_ghost_node : Tree.V.t -> t -> bool

    val is_ghost_edge : Tree.E.t -> t -> bool

    exception Ghost_node

    val get_graph_vertex : Tree.V.t -> t -> Tree.V.label