CHAPTER 2- FUNDAMENTAL HUMAN AND CIVIL RIGHTS AND FREEDOMS
Article 22
Imposing of a heavier punishment than the one prescribed by the law in effect at the time when the crime was committed shall be prohibited.
No one shall be held guilty for a crime on account of any act which did not constitute a crime under the law in effect at the time when it was committed.
The law eliminating or mitigating the punishment for the offence shall be retroactive.
The law prescribing or increasing liability shall not be retroactive.
EDIT.