network.addEdge("a", "b", 11);

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

  @Test
  public void forTree_breadthFirst_infinite() {
    Iterable<Integer> result =
        Traverser.forTree(fixedSuccessors(Iterables.cycle(1, 2, 3))).breadthFirst(0);
    assertThat(Iterables.limit(result, 8)).containsExactly(0, 1, 2, 3, 1, 2, 3, 1).inOrder();