open list
val rec map [a, b] (f: (consumes a -> b), consumes xs: list a): list b =
 match xs with
 | Cons { head = h; tail = t } ->
 let h' = f h in
 let t' = map (f, t) in
 Cons { head = h'; tail = t' }
 | Nil ->
 xs
 end