f s X X f X a X f X c X f s X X f X X c X 2.2 f s X X f X a X f X c X f s X X true f X c X f s X X true a 1 0 s 1 1 c 1 2 f 2 0 f 2 1 2 0 f s X X f X a X true a 1 1 s 1 2 c 1 2 f 2 0 f 2 1 2 0 NaTT certifiable-1.6