I got hired to be an expert witness to prove this very thing, with a twist: Person P wrote software A in language RS/S. Person P then wrote software B in langua

I got hired to be an expert witness to prove this very thing, with a twist: Person P wrote software A in language RS/S. Person P then wrote software B in language VB/B. The question was whether they had improperly used software A when they wrote software B (they had sold software A) And I could prove that they had! The key was that we could find vestiges of the A code in the B code in a way that wasn't just person P's own personal style. for example, software A would set a logging variable as it did some processing, and under specific conditions would actually write to a log file. Software B also had the logging variable, and set in the exact same way even though the underlying code was different and it was never written out. And there were date-stamped comments in software A, and the same comments were present in software B, even though software B was written long after software A.