.
                                b   bud  b
                             u      bud    u
                           d        bud       d
                         b          bud         b
                        u             b              u
                       d              u               d
                      b               d                 b
                      u            b   b               u
                     d            u     u              d
                      b           d      d      ­       b
                       u            b   b             u
                         d             u              d
                           b           d            b
                             u          b         u
                                d       u      d
                                         d