Iterable<Character> result = Traverser.forGraph(SINGLE_ROOT).breadthFirst(charactersOf("a"));

    assertEqualCharNodes(result, "a");
  }

  @Test
  public void forGraph_breadthFirst_emptyGraph() {
    assertThrows(
        IllegalArgumentException.class,
        () -> Traverser.forGraph(createDirectedGraph()).breadthFirst('a'));
  }

  /**