.
                                       O
                             w      wn      w
                           n          It            n
                         I             O              I
                        t            w  n             t
                       O            I     t             O
                       w           O   w            w
                        n           n    I             n
                          I           t   O             I
                            t          wn           t
                              O         I           O
                                 w      t       w
                                      n  ~  n