No no no, there's a difference from re-creating work from memory, and re-creating work while looking at an old copy. It is also possible to work back from firs

No no no, there's a difference from re-creating work from memory, and re-creating work while looking at an old copy. It is also possible to work back from first principles. Heck, many Free Software was written like that: by rewriting the proprietary parts, possibly one by one.