datatype t = A | B val swap = fn x : t => case x of A => B | B => A datatype u = C of t | D val out = fn x : u => case x of C y => y | D => A datatype nat = O | S of nat val is_two = fn x : nat => case x of S (S O) => A | _ => B val zero_is_two = is_two O val one_is_two = is_two (S O) val two_is_two = is_two (S (S O))