From: Pat Hayes <phayes@ihmc.us>

Date: Fri, 28 Oct 2005 14:07:15 -0500

Message-Id: <p06230914bf88134bf1df@[10.100.0.16]>

To: Herman ter Horst <herman.ter.horst@philips.com>

Cc: www-rdf-comments@w3.org

Date: Fri, 28 Oct 2005 14:07:15 -0500

Message-Id: <p06230914bf88134bf1df@[10.100.0.16]>

To: Herman ter Horst <herman.ter.horst@philips.com>

Cc: www-rdf-comments@w3.org

>Following the period during which I reviewed preliminary >versions of the RDF Semantics spec (on behalf of the >WebOnt WG) I investigated various aspects of the RDF >semantics, which led to two papers [1] [2] that contain >information that can be viewed as complementing the spec [3]. >In this message I include several comments relating to [3], >derived from [1] [2]. >In particular I discovered an error that remains >in the RDFS entailment lemma and that was missed earlier. Thank you for discovering this error. I have a comment on it, and on your proposed solution, below. This also has some bearing on your other comments. >=== > >A) In [3] decidability and complexity of RDFS entailment are >not considered. The completeness proof in [3] does not prove >decidability of RDFS entailment because the RDFS closure graphs >used are infinite. >In [2] it is proved that these infinite RDFS closure graphs >can be replaced by finite, partial RDFS closure graphs that >can be computed in polynomial time. > This is used in [2] >to prove that RDFS entailment is decidable, NP-complete, and >in P if the target RDF graph has no blank nodes. > >More specifically, a decision procedure for RDFS entailment >can be described as follows. > >Given: a finite set S of finite RDF graphs, a finite RDF graph G: >- form a merge M(S) of S >- add to M(S) all RDF, RDFS axiomatic triples, except those >including a container membership property rdf:_i >(this is a finite set of axiomatic triples) >- for each container membership property rdf:_i that appears >in either S or G, add the four (RDF, RDFS) axiomatic triples >that include rdf:_i; if S and G do not include a container >membership property, then add the four axiomatic triples >for just one container membership property >- apply rule lg to any triple containing a literal, in such >a way that distinct, well-formed XML literals with the same >value are associated with the same surrogate blank node >- apply rules rdf2 and rdfs1 to each triple forming a >well-formed XML literal or a plain literal, respectively >- apply rule rdf1, rule gl and the remaining RDFS entailment >rules until the graph is unchanged > >In [2] I call the graph H that is now obtained a partial >RDFS closure: it is finite (see [2] for the proof). > >It can be decided whether S RDFS-entails G by checking whether >H contains an instance of G as a subset or whether H >contains an XML-clash (see [2] for the proof). > >In this decision procedure, the entailment rules are interpreted >in a broader way than in [3]: blank nodes are allowed >in predicate position. See the following comment. > >=== > >B) There is an error in the RDFS entailment lemma [3]. >For example, the three RDF triples > >p subPropertyOf b . >b domain u . >v p w . > >(where b is a blank node) RDFS-entail the triple > >v type u . > >This RDFS-entailment cannot be found with the axiomatic >triples and entailment rules for RDFS. >These entailment rules are therefore not complete. Indeed. > >In [2] it is proved that this problem can be solved by extending >RDF graphs to allow blank nodes in predicate position. >The origin of the problem seems to be a 'mismatch' between >the notion of RDF graph and the semantics of RDF. >The semantics allows blank nodes to refer to properties, >while RDF graphs do not allow blank nodes in predicate >position. >It seems that if blank nodes would be allowed in predicate >position in RDF graphs, then the abstract syntax (i.e. >RDF graphs) would fit more closely with the semantics. Agreed. There is a similar 'mismatch', noted earlier by others, in that RDF syntax does not allow literals to occur as subjects, even though an RDF subject node may denote a literal value. That particular mismatch is what gave rise to the need for the lg and gl rules in the semantics document, as you know. I believe that all these problems would be eliminated, and the rule sets simplified, if we were to define the closure rules on an extended RDF syntax consisting of e-graphs, in which both these restrictions were removed, i.e. sets of e-triples, where an e-triple is <s,p,o> with s and o both a URIref, literal or bnode, and p a URIref or bnode. (It would be harmless to allow p to be a literal, but it is not necessary.) There would then be no need for the existential generalization rules lg and gl to be used when defining a closure, and all consideration of existential generalization can be restricted to checking simple entailment. These rules are needed at present only to provide a way to map from triples with literal objects: a b "l" . to put a 'corresponding' bnode in a subject position _:l type Literal . a b _:l . In e-graphs, this can be done directly: "l" type Literal . An XML clash is then simply an assertion of the form "l"^^XMLLiteral type Literal . where 'l' is not a legal XML string. Note that defining the closure rules on this extended syntax does not itself require extending RDF: the e-graph syntax could be used only internally to an inference engine, and theorems or deductions filtered to only allow correct RDF syntax, or modified by the addition of bnode-subject triples to express suitable conclusions. However, this generalization of the RDF syntax model does seem to be a sensible proposal, on independent grounds. BTW, allow me to express my wholehearted endorsement of your pD* semantic style for OWL, described in [2], which seems to retain the useful parts of the language, removes a number of highly unintuitive entailments, and greatly improves efficiency. This represents the way that OWL should have been defined as an extension of RDFS, in my view, and is a sounder basis for future work than the current OWL-DL spec. Pat Hayes > >[1] H. ter Horst, Extending the RDFS Entailment Lemma, >Proceedings 3rd International Semantic Web Conference (ISWC2004), >Springer LNCS 3298, pp. 77-91. > >The following is a revised and extended version of [1] > >[2] H. ter Horst, Completeness, decidability and complexity of >entailment for RDF Schema and a semantic extension involving the >OWL vocabulary, Journal of Web Semantics 3 (2005) pp. 79-115. >http://www.sciencedirect.com/science?_ob=IssueURL&_tockey=%23TOC%2312925%232005%23999969997%23606589%23FLA%23&_auth=y&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=814973d32693889bd7e4cf071c815f01 > >[3] http://www.w3.org/TR/rdf-mt/ -- --------------------------------------------------------------------- IHMC (850)434 8903 or (650)494 3973 home 40 South Alcaniz St. (850)202 4416 office Pensacola (850)202 4440 fax FL 32502 (850)291 0667 cell phayesAT-SIGNihmc.us http://www.ihmc.us/users/phayesReceived on Friday, 28 October 2005 19:07:29 UTC

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