Steve, I could make it work with extensions, but - and I had forgotten about that - one can only add new elements to the end of a sub-type, which may not be the right place from a semantical perspective.

So one would have to re-define such a type from scratch. The same applies for types where an element needs to be replaced and all the types on the "path" from the root element to such re-defined types. So I think one would proably end up with only a few non-redefined types. In addition such an approach puts the burden of dealing with types from two different namespaces onto the user which may not be desirable.

To cut a long story short, I also came to think that writing the file from scratch actually is the better way