Redirect edges so that target is the root of its subtree.
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);
    }
  }