.
                                   D
                            u    u c    u
                           c      k D      c
                          k        u          k
        ­                 D       c   k       D
                          u      D   u      u
                           c       c  k       c
                            k       D        k
                             D      u      D
                               u     c     u
                                   c k  c
                                      k