"kt.co",
          "ledger-enquirer.com",
          "members.blah.nl.",
          "pvt.k12.ca.us",
          "site.ac.jp",
          "site.ad.jp",
          "site.cc",
          "site.ed.jp",
          "site.ee",
          "site.fi",
          "site.fm",
          "site.geo.jp",
          "site.go.jp",
          "site.gr",
          "site.gr.jp",
          "site.jp",

   *     a
   *    /
   *   b   e <----> f
   *  / \ /
   * c   d
   * }
   */
  private static final SuccessorsFunction<Character> CYCLIC_GRAPH_CONTAINING_TREE =
      createDirectedGraph("ab", "bc", "bd", "ed", "ef", "fe");

  /**
   * A graph that is not a tree (for example, {@code h} is reachable from {@code f} via both {@code
   * e} and {@code g}) but is a valid input to {@link Traverser#forTree} when starting e.g. at node

   *     a
   *    /
   *   b   e <----> f
   *  / \ /
   * c   d
   * }
   */
  private static final SuccessorsFunction<Character> CYCLIC_GRAPH_CONTAINING_TREE =
      createDirectedGraph("ab", "bc", "bd", "ed", "ef", "fe");

  /**
   * A graph that is not a tree (for example, {@code h} is reachable from {@code f} via both {@code
   * e} and {@code g}) but is a valid input to {@link Traverser#forTree} when starting e.g. at node