add true x xs add and isNat x isList xs x Cons x xs isList Cons x xs isList xs isList nil true isNat s x isNat x isNat 0 true if true x y x if false x y y and true true true and false x false and x false false 2.2 add true x xs add and isNat x isList xs x Cons x xs isNat s x isNat x add true x xs isList xs add true x xs and isNat x isList xs isList Cons x xs isList xs add true x xs isNat x true add true x xs add and isNat x isList xs x Cons x xs true Failed! NaTT certifiable-1.6