a b x1 b b a x1 c b x1 b c c x1 2.2 c b x1 c x1 a b x1 a x1 c b x1 c c x1 true a b x1 a x1 true a 1 0 b 1 1 1 c 1 0 c 1 0 a 1 1 0 c b x1 c c x1 c b x1 c x1 true a 1 0 b 1 1 1 c 1 1 0 c 1 1 0 a 1 0 c b x1 b c c x1 NaTT certifiable-1.6