Local Open Scope pred.
Section extensions.
Context {CS: compspecs} {Espec: OracleKind}.
(* Scall *)

Lemma age_twin' {A B} `{HA: ageable A} `{HB: ageable B}:
forall (x: A) (y: B) (x': A),
level x = level y -> age x x' ->
exists y', level x' = level y' /\ age y y'.