*       of nodes that have been seen but not yet visited, that is, the "horizon").
   * </ul>
   *
   * <p><b>Examples</b> (all edges are directed facing downwards)
   *
   * <p>The graph below would be valid input with start nodes of {@code a, f, c}. However, if {@code
   * b} were <i>also</i> a start node, then there would be multiple paths to reach {@code e} and
   * {@code h}.

   */
  private static final SuccessorsFunction<Character> TWO_CYCLES_GRAPH =
      createDirectedGraph("ab", "ac", "bc", "cd", "da");

  /**
   * A tree-shaped graph that looks as follows (all edges are directed facing downwards):
   *
   * <pre>{@code
   *        h
   *       /|\
   *      / | \
   *     /  |  \
   *    d   e   g
   *   /|\      |
   *  / | \     |
   * a  b  c    f
   * }</pre>
   */

   */
  private static final SuccessorsFunction<Character> TWO_CYCLES_GRAPH =
      createDirectedGraph("ab", "ac", "bc", "cd", "da");

  /**
   * A tree-shaped graph that looks as follows (all edges are directed facing downwards):
   *
   * <pre>{@code
   *        h
   *       /|\
   *      / | \
   *     /  |  \
   *    d   e   g
   *   /|\      |
   *  / | \     |
   * a  b  c    f
   * }</pre>
   */