This is because tuples like `(T, U)` implement `PartialOrd` - this is actually useful. If you are going to implement `PartialOrd` for tuples, it makes sense to

This is because tuples like `(T, U)` implement `PartialOrd` - this is actually useful. If you are going to implement `PartialOrd` for tuples, it makes sense to implement it for 0-tuple too.