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