reconstruct represented tree as a DiGraph to
        preserve the current rootedness

def repr_tree(self):
        """
        reconstruct represented tree as a DiGraph to
        preserve the current rootedness
        """
        import utool as ut
        import networkx as nx
        repr_tree = nx.DiGraph()
        for u, v in ut.itertwo(self.values()):
            if not repr_tree.has_edge(v, u):
                repr_tree.add_edge(u, v)
        return repr_tree