HT's third example (was: Re: How to scope the note about D and override(E,D))

On Mar 14, 2011, at 9:39 AM, Henry S. Thompson wrote:

> 
> ht writes:
> 
>> A third example follows in the next message.
> 
> Consider the following set of 4 schema documents.
> ...
> 
> Algorithm O classifies this as OK starting from A, but not if starting
> - From B, C or D.


I agree that if one starts from B, C, or D one definitely
end up with contradictory declarations and thus with
a non-conforming schema.

I'm not entirely convinced that it's plausible to require
that the schema be accepted if one starts from schema
document A.   It depends, I think, on how aggressive the
processor wishes to be about detecting component 
identity.  

A step by step derivation of the meaning of schema(A)
in this case, working with the same notation introduced
in the phase-1 discussion of bug 6021, is at

http://lists.w3.org/Archives/Member/w3c-archive/2011Mar/0151.html

The commentary points out a few points in the calculation 
at which different processors might reach different
conclusions.

The algebraic derivation of schema(A) goes on for a 
long long time and involves several inclusions of
overridden versions of B, C, and D with different 
sets of children in the override.  Requiring that a 
processor recognize that override(E1,B) and
override(E1>E2, B) are the effectively the same
resource (where 'E1>E2' denotes the overlay of E2 
by E1) probably requires more intelligence than I
think the rest of the spec has been inclined to require
of a processor.  

So I think the status of the example is indeterminate
in the status quo and probably should remain so:  we
cannot provide a firmer footing here without a
complete revision of the foundations of the spec.
Unfortunately, I don't think we are in a position to do 
that now.  


-- 
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* C. M. Sperberg-McQueen, Black Mesa Technologies LLC
* http://www.blackmesatech.com 
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Received on Monday, 14 March 2011 22:25:59 UTC