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