[relax-ng] Control of the combinations of patterns

I am not sure that there is a very strong use case beyond this, but WXS
does pay lot of attention to the control of the derivations and
substitutions allowed by a schema.

With Relax NG, one can partially control the combinations done on a
start or named pattern: if I am writing a definition in a grammar which
will be reused by inclusion, if I write:

foo = bar

I allow grammars which will include this one to combine this definition
by choice or interleave while if I write:

foo = &bar

I forbid combinations by choice and if I write

foo = |bar

I forbid combinations by interleave.

Questions: 
 - would it be useful to be able to forbid both (similar to the notion
of "final" definition)? 
- would it be usefull to impose that a combination is applied (notion of
"abstract" definition)?

Eric
-- 
Curious about Relax NG? My book in progress is waiting for your review!
                                   http://books.xmlschemata.org/relaxng/
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Eric van der Vlist       http://xmlfr.org            http://dyomedea.com
(W3C) XML Schema ISBN:0-596-00252-1 http://oreilly.com/catalog/xmlschema
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