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You guys are all forgetting your history! At least one of {a, b, c},
in any order, is expressible in DTD, and therefore _a posteriori_ in
W3C XML Schema:
<!ELEMENT x ((a,((b,c?)|(c,b?))?)|(b,((a,c?)|(c,a?))?)|(c,((a,b?)|(b,a?))?))>
It's certainly not elegant, and it gets worse as the number of
alternatives goes up, but it's not impossible.
If order doesn't matter, and is therefore constrainable, then it's
much easier:
<!ELEMENT x ((a,b?,c?)|(b,c?)|c)>
ht
--
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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