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Re: Editorial changes in Section 2.5

From: Pat Hayes <phayes@ihmc.us>
Date: Sat, 28 Jan 2006 00:09:00 -0600
Message-Id: <p06230904c000b49671c5@[]>
To: jos.deroo@agfa.com
Cc: Enrico Franconi <franconi@inf.unibz.it>, RDF Data Access Working Group <public-rdf-dawg@w3.org>

>Pat Hayes wrote:
>>  it is broken, since it allows the bnodes in BGP to overlap with those
>>  in G'. Example:
>>  G' is
>>  :a :p _:b .
>>  :a  :q :e .
>>  and BGP is {?x :q _:b}. With this definition, this succeeds with x
>>  bound to :a, since there is a bnode variant of BGP, say {?x :q _:bb}
>>  , which satisfies the conditions; but the corresponding instance of
>>  BGP itself, when unioned with the scoping graph, is not entailed by
>>  the scoping graph.
>I take it that G is
>:a :p _:bbb .
>:a :q :e .

OK, though G does not play any role here.

>and that G' is
>:a :p _:b .
>:a :q :e .


>and that BGP is
>{?x :q _:b}

Yes. Notice that they share a blank node.

>and that BGP' is
>{?x :q _:bb}


>and that the instance S(BPG') is
>:a :q _:bb.


>and anyhow find that G simply entails (G' union S(BGP'))
>:a :p _:bbb.
>:a :q :e.
>simply entails
>:a :p _:b.
>:a :q :e.
>:a :q _:bb.

Yes, exactly. But it should not, because in this case G does NOT 
simply entail (G' union S(BGP)). So in this case, that basic graph 
pattern should NOT match with that solution. This answer is correct 
for BGP' , but it is not correct for BGP. Which is what matters, 
since the definition is supposed to be defining a match for BGP.

The point is that we introduced G' in order, partly, to be able to 
ensure that there were no 'accidental' bnode clashes between the G 
and the BGP (and in part to ensure that all the answer bindings used 
bnodes consistently with one another and with their pattern of useage 
in G.) The original phrasing, in which we simply said that (G entails 
(G union S(BGP))), was wrong, as Enrico noted, because G and BGP 
might accidentally share bnodes, so there is a need to standardize 
them apart. FUB suggested the directed merge trick to fix this; and 
then we introduced the scoping graph G', and noticed that since G' 
could be stipulated by definition to be standardized apart from all 
the BGPs, there was no need to use a directed merge, because now a 
simple union would do. But by requiring the bnode-separation 
condition to hold not between G' and BGP (as it should), but between 
G' and some variant BGP' of BGP, we have rendered this condition 
vacuous, since BGP itself can now share bnodes with G', which is 
exactly the situation we set up all this machinery to avoid in the 
first place.

I did not misread the definition: I suggest that y'all read through 
the above carefully, and think about it.


>Jos De Roo, AGFA http://www.agfa.com/w3c/jdroo/

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Received on Saturday, 28 January 2006 06:08:45 UTC

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