tion sufficient to validate a statute's prospective application under the Clause "may not suffice" to warrant its retroactive application. Usery v. Turner Elkhom Mining Co., 428 U.S. 1, 17, 96 S.Ct. 2882, 2893, 49 L.Ed.2d 752 (1976). FN19. Article I contains two Ex Post Facto Clauses, one directed to Congress (§ 9, cl