*     /  |  \
   *    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

   *     /  |  \
   *    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

    // Test setup fails on JDK9
    // java.security.KeyStoreException: Certificate chain is not valid
    // at sun.security.pkcs12.PKCS12KeyStore.setKeyEntry
    // http://openjdk.java.net/jeps/229
    // http://hg.openjdk.java.net/jdk9/jdk9/jdk/file/2c1c21d11e58/src/share/classes/sun/security/pkcs12/PKCS12KeyStore.java#l596
    val keystoreType = if (platform.isJdk9()) "JKS" else null