MutableNetwork<String, Integer> network = NetworkBuilder.directed().build();
    network.addEdge("a", "b", 11);

    Traverser.forTree(network); // Does not throw
  }

  @Test
  public void forTree_withUndirectedNetwork_throws() throws Exception {
    MutableNetwork<String, Integer> network = NetworkBuilder.undirected().build();
    network.addEdge("a", "b", 11);