It has the override attribute, which implements the override functionality of TR 9401. We need to define what happens if this isn't defined in the catalog. We could make override required. This is an issue. We may want to go beyond 9401, which makes it implementation-dependent.
Norm proposes a nesting catalog element.
The semantics are that it takes effect at the end of the catalog. This seems like a fallback, but it often isn't used that way. We could force the element to go at the end of the file. Norm has made a new issue for this. Try to just figure out what we call it to start with.