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   Re: [xml-dev] Subtyping in XML

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Without commenting in detail on your summary of your (and others')
thinking on this subject, for which thanks, I would like to point out
that derivation by restriction has very different properties wrt its
impact on processors which don't expect it than derivation by
extension does.  Substitution groups are somewhere in between.  That's
why I mentioned blocking _extension_ in the bit you quoted from me,
not blocking all derivations.

There's a whole rich and interesting topic I think of as "Designing
for Extensibility" wrt W3C XML Schema, and my thinking on this for
some time has been to focus on achieving this via the use of
'generous' abstract types and derivation by restriction, for just the
kind of reasons you cite.

  Henry S. Thompson, HCRC Language Technology Group, University of Edinburgh
          W3C Fellow 1999--2002, part-time member of W3C Team
     2 Buccleuch Place, Edinburgh EH8 9LW, SCOTLAND -- (44) 131 650-4440
	    Fax: (44) 131 650-4587, e-mail: ht@cogsci.ed.ac.uk
		     URL: http://www.ltg.ed.ac.uk/~ht/
 [mail really from me _always_ has this .sig -- mail without it is forged spam]


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