Andrew Tridgell recommends reading software patents, and describes how to do it right. "That first type of defence is really the one you want, it's called: non-infringement. And that is: 'we don't do that. The patent says X, we don't do X, therefore go away, sue someone else, it's not relevant for us'. That's the defence you want. [...] Next one, prior art: [...] Basically the argument is: somebody else did that before. It's a very, very tricky argument to get right. Extremely tricky, and it is the most common argument bandied about in the free software community. And if you see it in the primary defence against a patent, you should cringe because it is an extremely unsafe way of doing things."