a x1 b x1 a b x1 b c a x1 b x1 c x1 c b x1 a x1 2.2 a b x1 c a x1 c b x1 a x1 a x1 b x1 a b x1 a x1 a b x1 b c a x1 b x1 c x1 true b x1 c x1 a b x1 b c a x1 a b x1 a x1 c b x1 a x1 a x1 b x1 a b x1 c a x1 true a 1 1 41 b 1 1 41 c 1 1 0 c 1 1 0 a 1 1 2 b 1 1 1 c b x1 a x1 a x1 b x1 b x1 c x1 a b x1 b c a x1 NaTT certifiable-1.6