[atomic_dog]

UNITED STATES CONSTITUTION
Article I
Section 10
Clause 1: No State shall enter into any Treaty, Alliance, or Confederation; grant Letters of Marque and Reprisal; coin Money; emit Bills of Credit; make any Thing but gold and silver Coin a Tender in Payment of Debts; pass any Bill of Attainder, ex post facto Law, or Law impairing the Obligation of Contracts, or grant any Title of Nobility.

[F. Barnard]

The ex post facto prohibition applies to only to criminal law.