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