And let's say that our universe, our universe that we care about right over here, is the set of integers. So our universe is a set of integers. So I'll write U, capital U, is equal to the set of integers. And this is a little bit of an aside, but the notation for the set of integers tends to be a bold Z. And it's Z for Zoll from German for apparently integer. And the bold is this kind of weird looking, they call it blackboard bold. And it's what mathematicians use for different types of sets of numbers.