> data Vect : Nat -> Type -> Type where
>      Nil : Vect Z a
>      (::) : a -> Vect k a -> Vect (S k) a

> %name Vect xs, ys, zs

> append : Vect n a -> Vect m a -> Vect (n + m) a
> append xs ys
>     = case xs of
>            [] => ?bar_3
>            (x :: zs) => ?bar_4