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