Issue: key/keyRef and modularity

I would like to open an issue on key/keyRef and modularity.  I am concerned
that there are going to be problems with key/keyRef when you try to combine
independently-authored files using <include>.  For example,

<choice>
  <include href="foo.rng"/>
  <include href="bar.rng"/>
</choice

may fail when key names are used inconsistently in foo.rng and bar.rng, and
there's not much that the user can do about it.

Maybe <include> should be able to contain elements to rename keys?

<include href="foo.rng">
  <renameKey from="foo" to="bar"/>
</include>

or maybe <include> could have a flag to ignore keys in the included pattern.

Or maybe we need some explicit scoping for key names?

I don't have an answer yet, but I think this needs further consideration.

James