A mix in to me is a set of functionality that you can add to a class. It's more like aspect oriented programming or something like that where, which I'm not necessarily a huge fan of, but it's nice to be able to kind of compose functionality. Mixins are a way of doing composition with class inheritance. So let's move on to the next section. Metaclasses. You're talking about that you can register class existence with the Metaclass. What’s that specifically about? Nice.