When A and B are disjoint, you don't need the tag unless you for some reason you require that `Maybe<Maybe<T>> != Maybe<T>` holds true and don't like the collapsing semantics of unions where `Maybe<Maybe<T>> == Maybe<T>`

In practice, the cohabitants in Option/Result types are almost always disjoint. So you spend time wrapping/unwrapping Some/Ok/Err for no value add.