Depends on local laws. In the US, it's very tough to protect compilations of otherwise public facts. The real answer, however, is that if you had the funds and

Depends on local laws. In the US, it's very tough to protect compilations of otherwise public facts. The real answer, however, is that if you had the funds and it made good financial sense to challenge and pursue them, your whole perspective on the affair would change. Litigating such a messy case would be very, very expensive (again, in the US at least). One goal in licensing is to think about "how do I make violations clearcut and cheaper to enforce?" Your approach might require proving how they compiled their code together -- i.e., was it just coincidence that the two ended up looking similar? What if they coded their own VAT tax code but used every other piece of your code? Would that be enough to get around the GPL license? Just a couple examples of why such litigation would be so complex and expensive. You generally don't want to have to get into motives and process -- if you can't look at the two codebases and decide the case, it will be messy. I'd just keep more of your code proprietary and only permissively license the stuff with very broad appeal.