.
                                       F
                             a      a c      a
                          c         e F          c
                         e          a c             e
                        F             e                F
                       a              F                 a
                       c             a  c               c
                       e            e    F              e
                       F            a    c              F
                         a            e F             a
                           c            a             c
                               e        c         e
                                         e