intuition.
eapply constant_cost_le.
eapply constant_cost_bind; eauto.
eapply constant_cost_const.
omega.
Qed.

Theorem constant_cost_bind_pair :
forall (A  : Type)(B C: Set)
(f1 : A -> Comp B)(f2 : A -> B -> Comp C) c1 c2,
cost f1 c1 ->
cost (fun p => f2 (fst p) (snd p)) c2 ->
cost (fun a => Bind (f1 a) (f2 a)) (c1 + c2).