intuition.
apply constant_cost_pair_args in H0.
destruct H0.
destruct H0.
intuition.
eapply constant_cost_le.
eauto.
omega.
Qed.

Theorem constant_cost_compose_unary :
forall (A B C: Type)
(f1 : A -> B)(f2 : B ->C) c1 c2,
cost f1 c1 ->
cost f2 c2 ->
cost (fun a => f2 (f1 a)) (c1 + c2).