assertEqualCharNodes(traverser.depthFirstPreOrder('c'), "cdab");
    assertEqualCharNodes(traverser.depthFirstPreOrder('d'), "dabc");
  }

  @Test
  public void forGraph_depthFirstPreOrderIterable_cycle() {
    Traverser<Character> traverser = Traverser.forGraph(CYCLE_GRAPH);
    assertEqualCharNodes(traverser.depthFirstPreOrder(charactersOf("a")), "abcd");

   * a directory, no exception will be thrown and the returned {@link Iterable} will contain a
   * single element: that file.
   *
   * <p>Example: {@code Files.fileTraverser().depthFirstPreOrder(new File("/"))} may return files
   * with the following paths: {@code ["/", "/etc", "/etc/config.txt", "/etc/fonts", "/home",
   * "/home/alice", ...]}
   *
   * @since 23.5
   */

   * a directory, no exception will be thrown and the returned {@link Iterable} will contain a
   * single element: that file.
   *
   * <p>Example: {@code Files.fileTraverser().depthFirstPreOrder(new File("/"))} may return files
   * with the following paths: {@code ["/", "/etc", "/etc/config.txt", "/etc/fonts", "/home",
   * "/home/alice", ...]}
   *
   * @since 23.5
   */