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