.

                                     B
                           u      u  l        u
                         l          l  e            l
                       l              t                l
                      e              B                e
                      t               u                  t
                     B            l       l              B
                      u            e     t              u
                        l               B               l
                          l             u              l
                            e           l           e
                                 t       l       t
                                         e
                                         t