RE: Review of RIF Core entailment in SPARQL draft

> On behalf of the RIF working group I reviewed the RIF Core entailment
> section of the current SPARQL 1.1 Entailment Regimes draft [1]. Find
> my comments below. Please note that these comments are my personal
> comments and have not been discussed within the RIF working group.

Thanks Jos, the comments were very informative.  I have some follow-up that is not meant to 
be a formal reply (as many of the errors you pointed out below will need to be addressed)
, however, but is meant to elucidate the conversation regarding some of the more difficult
issues of safety and import closure (i.e., the graphs that are considered).

The other, more substantive portions, will probably be addressed in a formal response.

> Major comment (section 7.2):
> I did not understand the definition of Query Answers. It is unclear
> what the combination is for which RIF-Simple entailment is checked. In
> particular, which are the RDF graphs that are considered? Is this just
> the scoping graph or also the graphs imported by the rule set (the
> text in the "Legal Graphs" row seems to sanction such imports).

Yes, this was not considered at the time (the RDF graphs imported by the ruleset) and this
will probably need to be incorporated into the definition of definition of query answers.

>..snip..

> Further comments:
> - Sec 7.2, 1st editorial note: I wanted to see if I can answer the
> question, but could not find any definition of "answer set" in the
> draft

That should say set of query answers or solutions (as defined by the entailment regime), rather than 'answer set'.  The general question for which feedback is being solicited in that editorial note is regarding the restrictions on the imported RIF document necessary to 'guarantee that the set of triples obtained by instantiating BGP with each solution is uniquely specified up to RDF graph equivalence' and ensure finite answers (i.e., query safety).  What would such restrictions be short of strong safety (which is to restrictive)?

Even with modifications to the definition of query answers such that RDF graphs imported from the referenced ruleset are considered, the question would still be the same, I imagine.

> - Sec 7.3: I do not understand the relation between the text in this
> section and RIF Core entailment in SPARQL. I see implementation hints
> about RDFS and OWL 2 RL; it seems to me these hints should be included
> in the respective RDFS and OWL 2 RL sections.
> - Sec 7.4, 2nd sentence: I cannot parse '(with possibly infinite)'.

This should just be 'infinite'.  I.e., built-in functions and predicates that are infinite.  

> - Sec 7.4, 2nd par: it is not clear why stratification is discussed
> here, since RIF Core has not negation. In general, I find the
> paragraph a bit confusing. I think it should be rewritten.
>..snip..
> Discussion points:
> - I would suggest to remove the rif:imports triple from the scoping
> graph, since it may lead to unexpected inferences.

Do you have any examples of how this might happen?

-- Chime

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