.
                                        b
                     ­            a    a d     a
                               d      b a       d
                              b        d            b
          ­                   a        b  a          a
                              d       d   b         d
                              b       a   d         b
                               a       b  a         a
                                d        d         d
                                  b      b        b
           ­                          a    a     a
                                           d