intuition.
eapply cost_le.
eapply (cost_compose f2 (fun a x => f3 (f1 a) x)).
eauto.
intuition.
eapply (cost_compose f1 (fun a => f3)).
eauto.
intuition.
eapply cost_const.
intuition.
eauto.
intuition.
omega.
Qed.

(* Specifications: *)
Theorem cost_pair_1 :
forall (A B : Type),
cost (@pair A B) 0.