Writing unit tests is pretty much standard practice these days. Otherwise, how would you make sure that your code does what you expect? Yet, some software is mission-critical and merely testing a few examples – or even randomized testing – is not enough. To reach higher levels of assurance, we need proof: mathematical, formal proof. This session will be based on an example from industry, where we successfully verified the core of a financial application. I will describe the core architecture of the system and the mathematical foundations behind the verification, including the classes of problems that we can (or cannot) discover with this approach.