Discuss the principle of non-retroactivity and its exceptions.

Criminal law is the state's most coercive instrument; the general principles are its constitution — the limits that make punishment law rather than power. Each principle below answers a distinct question: May a person be punished for not knowing the law (ignorantia)? May conduct be criminal without pre-existing law (nullum crimen/lex certa)? Must the punishment also be pre-fixed (nulla poena)? How are ambiguous penal texts read (strict construction)? Does prosecution survive the accused's death (actus personalis)? May law operate backwards (ex post facto)? May a person be tried twice (double jeopardy)? Who bears the burden of proof (presumption of innocence)? Together they encode the rule of law (Constitutional Law-I, Unit 3) in penal form — and the Constitution of Nepal (2015), Article 20, elevates most of them to fundamental rights.