*/
  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>

Duchess:  you'd better ask HER about it.'

  `She's in prison,' the Queen said to the executioner:  `fetch
her here.'  And the executioner went off like an arrow.

   The Cat's head began fading away the moment he was gone, and,
by the time he had come back with the Dutchess, it had entirely
disappeared; so the King and the executioner ran wildly up and down

   *       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

   */
  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>

Duchess:  you'd better ask HER about it.'

  `She's in prison,' the Queen said to the executioner:  `fetch
her here.'  And the executioner went off like an arrow.

   The Cat's head began fading away the moment he was gone, and,
by the time he had come back with the Dutchess, it had entirely
disappeared; so the King and the executioner ran wildly up and down