.

                                     D
                             u    u   t    u
                          t        y   D        t
                        y            u             y
                       D              t              D
                      u              y                u
                      t               D                 t
                      y           u     t              y
                      D          y     D             D
                      u           u    t              u
                        t              y               t
                         y            D             y
                          D           u           D
                             u         t         u
                                t       y       t
                                  y    D   y
                                        u
                                        t
                                        y