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