code: function redirect(v) {
      var edges = tree.inEdges(v);
      for (var i in edges) {
        var e = edges[i];
        var u = tree.source(e);
        var value = tree.edge(e);
        redirect(u);
        tree.delEdge(e);
        value.reversed = !value.reversed;
        tree.addEdge(e, v, u, value);
      }
    }
docs: Redirect edges so that target is the root of its subtree.