.
                                     F
                             a     ac     a
                           c       eF        c
                         e         ac          e
                        F           e            F
                        a        F    a         a
                         c       c     e         c
                          e       F   a        e
                            F      c  e       F
                              a      F      a
                                c    a     c
                                  e  c   e
                                      e