.
                                   B
                        e        e   s         e
                     s           t   B            s
                   t                e                 t
                  B                s                   B
                 e                  t                    e
                 s              B      e                s
                 t              e        s                t
                  B             t      B               B
                    e               e                 e
                        s           s              s
                            t        t          t
                                     B    
                                     e
                                     s
                                     t