f 0 x1 s 0 x1 d 0 x1 0 x1 d s x1 s s d x1 f s x1 d f x1 2.2 f s x1 d f x1 f s x1 f x1 d s x1 d x1 true f s x1 f x1 true d 1 0 s 1 1 1 f 1 0 0 1 0 d 1 0 f 1 1 0 f s x1 d f x1 false d s x1 d x1 true d 1 0 s 1 1 1 f 1 0 0 1 0 d 1 1 0 f 1 0 NaTT certifiable-1.6