b b c a b c x1 a b b c b c a x1 2.2 b b c a b c x1 b c b c a x1 b b c a b c x1 b c a x1 b b c a b c x1 b b c b c a x1 true b b c a b c x1 b b c b c a x1 false b b c a b c x1 b c a x1 false b b c a b c x1 b c b c a x1 false NaTT certifiable-1.6