how to specify what extends what in doctrine mappedsuperclass with xml mapping

I'm trying to figure out how the superclass mapping works in Doctrine2, and I have some examples to work with, but I've been using the xml mapping to create my Entities and then in turn the database schema. I found one example that showed an 'extends' parameter to the tag, but it does not appear to be supported in the current schema.

How do you go about telling the XML driver what classes should extend a mapped superclass?

SW