.
                                 B
                       a      a   n       a
                    n         k   B          n
                  k            a   n            k
                 B               k                B
                 a               B                 a
                 n               a                  n
                 k            n      k             k
                   B         B       a            B
                    a          n     k           a
                      n            B            n
                          k         a         k
                                     n
                                     k