[SideburnsOfDoom]

We seem to have yet another potential meaning here : https://news.ycombinator.com/item?id=47692261

> Cat, Dog and Bird don't have to inherit from the union, you can declare a union of completely random types, as opposed to saying "Animal has three subtypes, no more, no less"

"Animal has three subtypes" is more like the c# "sealed" modifier on a class, meaning that subtyping is not allowed. Except in this case I guess for three existing subtypes.