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