.
                                    B
                         ­   a     a b       a
                         b        y B          b
                        y         a b            y
                       B            y               B
                      a             B                a
                      b          a     b            b
                       y          y    B            y
                        B         a    b           B
                          a        y   B         a
                              b     a b     b
                                       Y