() -> Traverser.forTree(createDirectedGraph()).breadthFirst('a'));
  }

  @Test
  public void forTree_breadthFirstIterable_emptyGraph() {
    assertEqualCharNodes(
        Traverser.forTree(createDirectedGraph()).breadthFirst(charactersOf("")), "");
    assertThrows(
        IllegalArgumentException.class,
        () -> Traverser.forTree(createDirectedGraph()).breadthFirst(charactersOf("a")));
  }