f f a a x f x f f a f a a a 2.2 f f a a x f f a f a a a f f a a x f a f a a f f a a x f x f f a f a a a true f f a a x f x f f a f a a a true 2 1 a 0 2 1 f 2 0 1 0 0 1 0 1 0 0 2 1 0 f 2 0 1 1 0 1 0 1 1 0 2 0 0 f f a a x f x f f a f a a a f f a a x f a f a a false f f a a x f f a f a a a false NaTT certifiable-1.6