intuition.
eapply (@expected_poly_time_compose_simp _ _ _ _ (fun n a => snd a)); eauto.
eapply expected_poly_time_snd.
Qed.

Theorem expected_poly_time_compose_0_binary :
forall (A B C : nat -> Type)(f1 : forall n, A n)(f2 : forall n, B n)(f3 : forall n, A n -> B n -> C n) ,
efficient _ f1 ->
efficient _ f2 ->
efficient _ f3 ->
efficient _ (fun n => f3 n (f1 n) (f2 n)).