Section BIJECTION.
Variables A B : Type.
Variable JA: Join A.
Variable PA: Perm_alg A.
Variable ag: ageable A.
Variable bijAB: bijection A B.
Variable asa : Age_alg A.
Existing Instance PA.
Instance agB : ageable B := (ag_bij _ _ ag bijAB).
(*  Instance PA_B:  @Perm_alg B (Join_bij _ _ _ bijAB ) := @Perm_bij A JA PA B bijAB. *)

Theorem asa_bijection : @Age_alg B (Join_bij _ _ _ bijAB) agB.