= = 0 = x = 0 1 = 9 1 = 0 x = 0 1 x = 9 1 1 1 = 0 1 1 x = 9 1 1 ? ? 9 ? x ? 0 x = 9 ? 9 1 ? 0 1 1 ? 0 1 x s 0 1 ? 9 1 1 s s 9 s x s 0 x