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Ex post facto means to do something after the fact. Article One of the Constitution prevents the Congress from taking action after the fact. In other words Congress cannot pass a law and have the law apply to people who may have broken the law before the law was passed.
(Click on citations for more information)
Article 1, Section 9, Cluase 3
Article 1, Section 10, Clause 1
No Bill of Attainder or ex post facto Law shall be passed.
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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.