*
   * {@snippet :
   *   a
   *  / \
   * b   c
   *  \ /
   *   d
   * }
   */
  private static final SuccessorsFunction<Character> DIAMOND_GRAPH =
      createDirectedGraph("ab", "ac", "bd", "cd");

  /**
   * Same as {@link #DIAMOND_GRAPH}, but with an extra c->a edge and some self edges:
   *
   * {@snippet :
   *   a<>
   *  / \\
   * b   c
   *  \ /
   *   d<>
   * }
   *

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