assertThrows(
        IllegalArgumentException.class,
        () -> Traverser.forGraph(createDirectedGraph()).breadthFirst('a'));
  }

  /**
   * Checks that the elements of the iterable are calculated on the fly. Concretely, that means that
   * {@link SuccessorsFunction#successors(Object)} can only be called for a subset of all nodes.
   */
  @Test
  public void forGraph_breadthFirstIterable_emptyGraph() {

You know the song, perhaps?'

  `I've heard something like it,' said Alice.

  `It goes on, you know,' the Hatter continued, `in this way:--

            "Up above the world you fly,
            Like a tea-tray in the sky.
                    Twinkle, twinkle--"'

Here the Dormouse shook itself, and began singing in its sleep
`Twinkle, twinkle, twinkle, twinkle--' and went on so long that

     *   lock.lock();
     * }
     * operation();
     * for (Lock lock : locks) {
     *   lock.unlock();
     * }
     *
     * If we only held the int[] stripes, translating it on the fly to L's, the original locks might
     * be garbage collected after locking them, ending up in a huge mess.
     */
    @SuppressWarnings("unchecked") // we carefully replaced all keys with their respective L's

    assertThrows(
        IllegalArgumentException.class,
        () -> Traverser.forGraph(createDirectedGraph()).breadthFirst('a'));
  }

  /**
   * Checks that the elements of the iterable are calculated on the fly. Concretely, that means that
   * {@link SuccessorsFunction#successors(Object)} can only be called for a subset of all nodes.
   */
  @Test
  public void forGraph_breadthFirstIterable_emptyGraph() {

   * while iteration is in progress.
   *
   * <p>The returned {@code Iterable} can be iterated over multiple times. Every iterator will
   * compute its next element on the fly. It is thus possible to limit the traversal to a certain
   * number of nodes as follows:
   *
   * <pre>{@code
   * Iterables.limit(Traverser.forGraph(graph).breadthFirst(node), maxNumberOfNodes);
   * }</pre>
   *

You know the song, perhaps?'

  `I've heard something like it,' said Alice.

  `It goes on, you know,' the Hatter continued, `in this way:--

            "Up above the world you fly,
            Like a tea-tray in the sky.
                    Twinkle, twinkle--"'

Here the Dormouse shook itself, and began singing in its sleep
`Twinkle, twinkle, twinkle, twinkle--' and went on so long that