intuition.
eapply (@expected_poly_time_compose_binary _ _ _ _ _ _ BVxor); eauto.
eapply expected_poly_time_bvxor.
Qed.

Theorem expected_poly_time_eqb_bool_f :
forall (A : nat -> Type)(f1 : forall n, A n -> bool)(f2 : forall n, A n -> bool),
efficient _ f1 ->
efficient _ f2 ->
efficient _ (fun n (a : A n) => eqb (f1 n a) (f2 n a)).