f a f a x f a f f a a f a x 2.2 f a f a x f f a a f a x f a f a x f a a f a f a x f a f f a a f a x true f a f a x f a f f a a f a x false f a f a x f a a false f a f a x f f a a f a x false NaTT certifiable-1.6