*     /  |  \
   *    d   e   g
   *   /|\      |
   *  / | \     |
   * a  b  c    f
   * }
   */
  private static final SuccessorsFunction<Character> TREE =
      createDirectedGraph("hd", "he", "hg", "da", "db", "dc", "gf");

  /**
   * Two disjoint tree-shaped graphs that look as follows (all edges are directed facing downwards):
   *
   * {@snippet :
   * a   c
   * |   |
   * |   |
   * b   d

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