a a x b b x b b a x a b b x 2.2 b b a x b b x a a x b x b b a x b x b b a x a b b x a a x b b x true a a x b b x b b a x a b b x b b a x b x a a x b x b b a x b b x true a 1 1 7721 b 1 1 7720 a 1 1 0 b 1 1 0 a a x b b x b b a x a b b x NaTT certifiable-1.6