.
                                     S
                           p       p r       p
                        r           i n          r
                       i             g              i
                      n            S                n
                     g            p   r             g
                     S           i     n             S
                      p          g     S            p
                       r          p    r            r
                         i          i  n           i
                            n        g         n
                                 g    ~     g