f x y cond and isNat x isNat y x y cond tt x y f s x s y isNat s x isNat x isNat 0 tt and tt tt tt and ff x ff and x ff ff 2.2 isNat s x isNat x f x y isNat y f x y isNat x f x y cond and isNat x isNat y x y cond tt x y f s x s y f x y and isNat x isNat y true cond tt x y f s x s y f x y cond and isNat x isNat y x y true Failed! NaTT certifiable-1.6