.
                                    D
                         ­  o     o  w      o
                       w        n  D          w
                      n             o              n
                     D             w               D
                     o              n                o
                     w          D     o           w
                      n         w      n          n
                       D         D     o          D
                        o          w  n          o
                           w        D         w
                               n     o      n
                                      w
                                       n