.

                                     S
                           e      e   r       e
                       r          v   i            r
                     v              c                 v
                    i                e                   i
                   c                S                    c
                   e                e                     e  
                   S             r       v               S
                    e            i        c              e
                      r            e    S              r
                        v             e               v
                          i            r             i
                             c        v          c
                                e     i       e
                                       c
                                       e