From: Peter F. Patel-Schneider <pfps@research.bell-labs.com>

Date: Fri, 20 Dec 2002 12:35:53 -0500 (EST)

Message-Id: <20021220.123553.28979011.pfps@research.bell-labs.com>

To: herman.ter.horst@philips.com

Cc: www-webont-wg@w3.org

Date: Fri, 20 Dec 2002 12:35:53 -0500 (EST)

Message-Id: <20021220.123553.28979011.pfps@research.bell-labs.com>

To: herman.ter.horst@philips.com

Cc: www-webont-wg@w3.org

From: herman.ter.horst@philips.com Subject: Abstract Syntax and Semantics: review comments Date: Fri, 20 Dec 2002 16:47:18 +0100 > Abstract Syntax and Semantics: > review comments about version of 19 December [...] > - Correspondence between OWL DL entailment and OWL Full entailment. > The Bristol consensus on semantics already expressed the need for a > characterization of the situation that DL and Full (in those days, fast > and large) entailment coincide [2]. There is now an informative appendix > with a brief sketch in this direction. > I do not understand how the theorem given there follows from the lemma. > It does not seem to follow from the statement of the lemma alone: if I is > an > OWL Full interpretation that satisfies K, then it needs to be proved that > I satisfies C. Getting another OWL DL interpretation I' that > satisfies K, shows that I' satisfies C. And then? > > Until we can really view the lemma and the theorem and its converse > as proved, we do not really know about the correspondence between > OWL DL entailment and OWL Full entailment. The theorem follows from the proof of the lemma, namely that the mapping between OWL-interpretations and OWL/DL-interpretations is uniform for a given separated vocabulary. I will strengthen the statement of the lemma accordingly. > - The entailment definitions in the document are theoretical, > in terms of models. This is as it should be. > It would be useful to have more practically useful, triple-based > characterizations of entailment, as in the RDF Semantics document [3]. The closure characterizations in the RDF semantic document are non-normative and prone to bugs. If this is true for RDF, which is very simple, think how difficult it would be to define the OWL-closure of an RDF graph. > As long as such characterizations remain unknown, > a sequence of simple sufficient conditions for entailment (i.e., simple > inference rules, formally stated, with proofs of validity) could > form a useful informative addition to the document in order to > familiarize people with the various kinds of OWL entailment. Again, the generation of such rules is very difficult and prone to error. People are willing to try, of course, but I would much rather spend my time in generating a proof procedure for OWL. > - It would also be nice to know how an OWL DL KB is characterized in > triple form. That is, what is an "OWL DL graph" as a special kind of > RDF graph? This is the subject of section 4 of the document. > - The document gives three definitions of OWL entailment (abstract, > DL en Full) but what is the normative definition of OWL entailment?. The document states that the direct model theory from section 2 is authoritative for OWL/DL ontologies. For OWL/Full ontologies, the RDFS-compatible model theory is authoritative. > The consensus summary said that in case of disagreement between > DL (fast) and Full (large) entailment, DL entailment is normative [2]. > In the new version, Peter introduced the > words "authorative" (for the direct model theory) and "secondary" > (for the RDFS-compatible model theory) in the document. > I think the word normative should be used. I can easily switch if this is the wording that should be used. > - Section 2.1: A small simplification is possible: use the two rules for > <annotation> also in <directive>, instead of their content. The rules are somewhat different, so reuse is not easily possible. > - 5.2: In the tables defining the cardinality restrictions: > instead of card({v:<u,v>...) make it completely explicit with the set > from which v can be taken: card({v elementOf ...:<u,v>...) This is not needed as card({v:<u,v> in EXT(...)}) delimits the set appropriately. > Herman ter Horst > Philips Research peterReceived on Friday, 20 December 2002 12:36:04 UTC

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