.
                                       B
                                u     ut      u
                              t        te          t
                             t          r             t
                            e        c   u           e
                            r        p    B          r
                             c        u   t           c
                             u         t  e          u
                               p         r           p
                                B        c         B
                                  u      u       u
                                     t     p     t
                                        t      t