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