.
                                    F
                             a    ac     a
                           c      eb       c
                          e       oo        e
                         b         k          b
                         o      F    a       o
                          o     c     e       o
                          k      b   o       k
                           F      o  k      F
                             a      F      a
                               c    a     c
                                 e  c  e
                                     e