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Review of RIF Core entailment in SPARQL draft

From: Jos de Bruijn <jos.debruijn@gmail.com>
Date: Tue, 27 Jul 2010 17:34:27 +0200
Message-ID: <AANLkTi=CMULJTyHHA9c-jvznbdK3zW4CWc05Cv+pmog5@mail.gmail.com>
To: public-rdf-dawg-comments@w3.org
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.

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).

Errors:
- Sec 7, 2nd par, 1st sentence: the phrase "...satisfiability and
entailment, each of which define how a common RIF-RDF interpretation
is a model of a combination" is incorrect. Satisfiability defines when
(not how) an interpretation is a model; entailment does not define
models.
- Sec 7, 1st par::
  * It is claimed here that entailment is checked wrt. a
Skolemization of the scoping graph. This contradicts the definition of
query answers in Sec. 7.2.
  * It is claimed that only combinations with one RDF graph are
considered. This seems to contradict the definition of Legal Graphs in
Sec. 7.2.
- Sec 7.2, def query answers: rif:imports is not a predicate, it is a CURIE
- Sec 7.2, "RIF-Core conformance is defined with respect to safe rules
and so this entailment regime is defined with respect to combinations
formed from safe RIF documents.": Not true. This entailment regime is
defined with respect to safe RIF document because it is defined as
such, in the row "Legal Graphs" of the table.
- Sec 7.4, 2nd par: cyclic references *do not* rule out the existence
of a unique minimal model. I believe that every RIF Core document has
a single unique minimal Herbrand model. Strong safety can, in
addition, guarantee finiteness (when not considering the datatypes).
- Sec 7.4, 3rd par: RIF models are necessarily infinite, since the
domain includes includes all integers.

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
- 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)'.
- 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.

Discussion points:
- I would suggest to remove the rif:imports triple from the scoping
graph, since it may lead to unexpected inferences.


[1] http://www.w3.org/TR/sparql11-entailment/#id35811294
Received on Tuesday, 27 July 2010 15:35:20 GMT

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