Parameter unsquash : rmap -> (nat * rmap').

(* Specifications: *)
Axiom rmap_level_eq: @level rmap _ = fun x => fst (unsquash x).
Axiom rmap_age1_eq: @age1 _ _ =
fun k => match unsquash k with
| (O,_) => None
| (S n,x) => Some (squash (n,x))
end.