a x1 x1 a b x1 b c a a x1 c b x1 x1 c c x1 b x1 2.2 a b x1 a a x1 a b x1 a x1 a b x1 c a a x1 true a b x1 a x1 a b x1 a a x1 true Failed! NaTT certifiable-1.6