Ltac do_map_arg :=
match goal with |- ?a = ?b =>
match a with context [map ?x _] =>
match b with context [map ?y _] => replace y with x; auto end end end.

Lemma level_age_fash:
forall m m': rmap, level m = S (level m') -> exists m1, age m m1. (* /\ comparable m1 m'. *)