.

                                       S
                                t    t   r     t
                             r       a  w     r
                            a           b           a
                          w            e             w
                          b             r               b
                         e              r                 e
                         r          y       S             r
                         r           t        r             r
                        y             r     a              y      
                        S                w                 S
                         t                b                  t
                          r              e                  r  
                           a             r                a
                            w           r              w
                                b        y          y
                                    e     r      r