Section BIJECTION.
Variable A B : Type.
Variable ag: ageable A.
Variable bijAB: bijection A B.
Let levelB (x:B) : nat :=
level (bij_g _ _ bijAB x).
Let age1B  (x: B) : option B :=
match age1 (bij_g _ _ bijAB x) with
| Some y => Some (bij_f _ _ bijAB y)
| None => None
end.
Let ageB (x y: B) :=age1B x = Some y.

Lemma ag_bij_facts : ageable_facts B levelB age1B.
Definition ag_bij : ageable B :=
mkAgeable B levelB age1B ag_bij_facts.