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entailment review - part 1

From: Axel Polleres <axel.polleres@deri.org>
Date: Tue, 5 Jan 2010 14:40:23 +0000
Message-Id: <20719265-31F8-47F8-9DE0-BB2CBE024E75@deri.org>
To: SPARQL Working Group <public-rdf-dawg@w3.org>
I am still catching up with Entailment and didn't complete the whole review in time...
... so, attached part 1, which essentially goes up to RDFS entailment only, didn't have 
time yet to take a closer look to the OWL parts.

None of my comments should really hold up publication, I think the document is overall in 
very good shape. 

However, actually some of my questions (comment 6) and 9) ) refer to the definitions taken from the current 
query spec and might need further rethinking/discussion of those.

Details in the attached file.


review http:http://www.w3.org/2009/sparql/docs/update-1.1/Overview.xml
CVS version 02/01/2010

Change summary:

The first public working draft defined the semantics of SPARQL queries under RDF and RDFS entailment. In this public working draft the RDF and RDFS entailment regimes have been changed to use a skolemized version of the queried RDF triples to limit the possible answers to a finite set of answers. This prevents non-local effects that caused additional results for existing triples from unrelated newly added triples that contain new blank nodes. This draft further adds an entailment regime for OWL Direct Semantics, which covers the OWL 2 DL, EL, and QL Profiles.


Detailed Comments:

1) (non-critical) Suggest to add a note in the Introduction: in future/final version of the document, only SPARQL 1.1/Query should be mentioned and no longer the "first SPARQL specification". SPARQL 1.1/Query also only talks about simple entailment, so all you say here about the old version will remain valid if you consistently only refer to the new SPARQL 1.1/Query document, I suppose.


2) "two Turtle documents which differ only by re-naming their node identifiers will be understood to be equivalent."

-I suggest to rather use "blank node identifier" than "node identifier"->

"two Turtle documents which differ only by re-naming their blank node identifiers will be understood to be equivalent."

3)
"In the case of any difference, the SPARQL Query Language definitions are the normative ones."
-not sure, as I am not native, but I think that should be plural->
"In the case of any differences, the SPARQL Query Language definitions are the normative ones."


4) "The term RDF-L denotes the set of all RDF Literals, RDF-B the set of all blank nodes in RDF graphs"

Hmmm, why "in RDF graphs" and in which graphs? BTW, this question also applies to Query as well.

5) "The web ontology language OWL allows for even more inferences."

As in other W3C docs, I suggest to consistently capitalise "Web".

* Another remark which is not critical, but maybe we should re-discuss it at some point ... BGP extension says that entailment regimes must specify:
 - well-formed graphs
 - SG must be unitquely specified
 - entailment relation
 - finiteness condition for answers
 - handling of inconsistent graphs 

It doesn't *actually* say that it should define restrict "which qeries are legal", does it? I anyway don't think that the definition of BGP extension 
does preclude such restrictions, but it isn't actually required by the original definition. 


6) This remark might be overshooting (at leat for this WD), but:

"The scoping graph, SG, corresponding to any consistent active graph AG is uniquely specified and is E-equivalent to AG."
[...]
"All entailment regimes specified here use the same definition of a scoping graph as given in SPARQL 1.0. Thus, the required equivalence is immediate."

I am a bit worried that *actually* the definition of the scoping graph as given in SPARQL 1.0 is *NOT* uniquely specified, since it obviously doesn't 
uniquely determine the blank nodes. Not sure whether this is really an issue, but it seems a bit awkward.

Maybe the condition should be weakened to something like 

"The scoping graph, SG, corresponding to any consistent active graph AG is uniquely (except blank node identifiers) specified and is E-equivalent to AG."

Not really ideal either, but better than before?
If we agree on that change, we can include that with a remark to ask for comments?


7) "We explain these restrictions in greater detail in the following section."
better: 
"We explain these restrictions in greater detail in the following sections."

I.e., there are different restrictions for the different entailment regimes in the following sections.

8) "Thus, also the following solution mappings are possible solutions:

   &mu;4 : s -> ex:a1, o -> _:c3,"

 Is this solution really possible? doesn't it violate (at least along with &mu1;) condition 3. ? 

9) Can we add an intuitive explanation, why for (C2) the restriction is made to subjects? Im mean, yes it is sufficient to achieve finiteness. Looking back through the mails, I see there was some discussion to change this restriction, cf.
http://lists.w3.org/Archives/Public/public-rdf-dawg/2009JulSep/0395.html
but what actually speaks against just reformulating even stricter to the vocabulary of the scoping graph only, i.e.

 "(C2) Each s,p,o in a triple ( s, p, o ) in P(BGP) occurs in the scoping graph."

?

That would be the first of the alternative choices in the Editorial note:
"Each subject s of a triple ( s, p, o ) in P(BGP) occurs in the scoping graph."
right?


10) the previous comment likewise apply to the RDFS Entailment Regime.

Received on Tuesday, 5 January 2010 14:40:57 GMT

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