datatype list a = Nil | Cons of a * list a

val rec append : t ::: Type -> list t -> list t -> list t = fn t ::: Type => fn ls1 => fn ls2 =>
        case ls1 of
            Nil => ls2
          | Cons (h, t) => Cons (h, append t ls2)

(*val rec ones : list int = Cons (1, ones)*)
val rec ones : unit -> list int = fn () => Cons (1, ones ())