The United States Constitution s "ex post facto" laws. Two clauses in the constitution prohibit "ex post facto". Article 1 Section 9 of the U.S. Constitution states: 'No Bill of Attainder or ex post facto Law shall be passed,'. Section 10 says: '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. . .'