a__f f a a__f g f a mark f X a__f mark X mark a a mark g X g X a__f X f X 2.2 mark f X mark X a__f f a a__f g f a mark f X a__f mark X true mark f X mark X true a 0 0 a__f 1 0 f 1 1 1 mark 1 1 0 mark 1 0 a__f 1 0 g 1 0 mark f X a__f mark X false a__f f a a__f g f a false NaTT certifiable-1.6