valueGraph.putEdgeValue("a", "b", 11);

    assertThrows(IllegalArgumentException.class, () -> Traverser.forTree(valueGraph));
  }

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

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

    valueGraph.putEdgeValue("a", "b", 11);

    assertThrows(IllegalArgumentException.class, () -> Traverser.forTree(valueGraph));
  }

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

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