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Re: complexType : extension of a sequence by a choice ?

From: <noah_mendelsohn@us.ibm.com>
Date: Sun, 8 Feb 2004 19:11:30 -0500
To: ht@inf.ed.ac.uk (Henry S. Thompson)
Cc: "'Bruno Chatel'" <bcha@chadocs.net>, "Michael Kay" <mhk@mhk.me.uk>, xmlschema-dev@w3.org
Message-ID: <OF17E1C63D.9A14919F-ON85256E35.00009FE6@lotus.com>

Henry Thompson writes:

>> Allowing one 'all' to be extended by 
>> (merged with) another would lead
>> to violations of the Extension invariant.

>> As it stands, there is an invariant for extension, just as 
>> there is for restriction, namely

>> Extension: Consider a type definition DE derived by extension from a
>> base type definition B.  Then any sequence of EIIs valid per DE
>> contains an initial sub-sequence valid per B.

>> [...]

Indeed.  That's the right invariant for sequences, but not I think for 
"all". 

Just as "all" is already different in many respects from other content 
models, I had always assumed that when and if we allow extension of an all 
group, we would not do it in an "all-specific" manner.  Specifically:  I 
assumed that we would allow an "all" group to be an extension of another 
"all" group if and only if the extension subsumed the base.  Conversely 
for restriction.  Whether we would require you to restate the original 
model in the extension, or only state the additions, seems to me to be 
syntactic sugar.   I don't much care, but would lean slightly toward 
explicitly providing only the new content.

I >think< we've actually had some user requests for such extension of 
"all", but unless Michael Kay is making such a request now (I don't think 
he is), I can't immediately find the reference.  I think it's a nice to 
have, but have no personal urgent need for such a new feature.  In any 
case the fact that it would violate the invariant quoted above doesn't 
particularly bother me.

--------------------------------------
Noah Mendelsohn 
IBM Corporation
One Rogers Street
Cambridge, MA 02142
1-617-693-4036
--------------------------------------
Received on Sunday, 8 February 2004 19:15:11 GMT

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