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Re: choice group particel whose {particles} is empty

From: Henry S. Thompson <ht@cogsci.ed.ac.uk>
Date: 26 Sep 2002 18:10:44 +0100
To: "Stanley Guan" <stanley.guan@oracle.com>
Cc: <xmlschema-dev@w3.org>
Message-ID: <f5badm44qy3.fsf@erasmus.inf.ed.ac.uk>

"Stanley Guan" <stanley.guan@oracle.com> writes:

> My main question is how useful choice B is?  Is it main use
> is like what I said -- "negative particle"?

Yes, but although that's the only _use_ one can imagine, that's not
why we allowed it -- rather because it seemed on balance better to
keep all the groups consistent, and allow no particles, than to
special-case <choice>.

The analogy I usually refer to is that and() (that is, boolean 'and',
no arguments) is always true, but or() is always false.  Compare
<sequence/> and <choice/>.

> > If you mean that a complex type definition which _has_ such a choice
> > as its content model is emptiable, then yes,
> >  A ---> emptiable, B -x-> emptiable
> Yes, this is what I mean.

Good, then I think that's all clarified.

  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/
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Received on Thursday, 26 September 2002 13:10:45 UTC

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