The term is used in situations where the
law (statutory, civil, or regulatory) is changed or reinterpreted, affecting acts committed before the alteration. When such changes make a previously committed lawful act now unlawful in a retroactive manner, this is known as an
ex post facto law or retroactive law. Because such laws punish the accused for acts that were not unlawful when committed, they are rare, and not permissible in most legal systems. Conversely, a form of retrospective law commonly called an
amnesty law may decriminalize certain acts. A
pardon has a similar effect, in a specific case instead of a class of cases. An
in mitius change may alleviate possible consequences for unlawful acts (for example, by replacing the death sentence with lifelong imprisonment) retroactively. Finally, when a previous law is
repealed or otherwise nullified, it is no longer applicable to situations to which it had been, even if such situations arose before the law was voided; this principle is known as
nullum crimen, nulla poena sine praevia lege poenali. ==Software development==