intuition.
eapply (@expected_poly_time_compose_0_binary _ _ _ a b (fun n a b => Bind a b)); eauto.
eapply expected_poly_time_bind.
Qed.

Theorem expected_poly_time_bind_f :
forall (A : nat -> Type)(B C : nat -> Set) (b : forall n, A n -> Comp (B n)) (c : forall n, A n -> B n -> Comp (C n)),
efficient _ b ->
efficient _ c ->
efficient _ (fun n a => (Bind (b n a) (c n a))).