Règle correspondante
Practice Relating to Rule 101. The Principle of Legality
Burundi’s Penal Code (2009), which includes a chapter on war crimes, states:
No offence can be punished by penalties which were not provided for by law before the offence was committed.
However, in the case of concurrence of two criminal laws, a former [law] which was in force when the offence was committed, and another [law] which was enacted since the [commission of the] offence and before a final judgement has been rendered, the new law shall only be applied if it imposes a less severe penalty. 
Burundi, Penal Code, 2009, Article 4.