.
                                     B
                             a      a t       a
                          t          t l           t
                         t           e s            t
                        l          B    a           l
                        e         t       t          e
                         s         l      e          s
                         B         s    B          B
                          a         a    t          a
                           t          t   l          t
                             t          e          t
                                l       s        l
                                    e   •    e
                                         s