intuition.
eapply (@expected_poly_time_compose_binary _ _ _ _ _ _ (fun n b c => pair b c)); eauto.
eapply expected_poly_time_pair.
Qed.

Theorem expected_poly_time_bvxor_f :
forall (A : nat -> Type)(f1 : forall n, A n -> Bvector n)(f2 : forall n, A n -> Bvector n),
efficient _ f1 ->
efficient _ f2 ->
efficient _ (fun n (a : A n) => BVxor n (f1 n a) (f2 n a)).