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Suggestions for response to: Some comments on OWL Reference

From: Guus Schreiber <schreiber@cs.vu.nl>
Date: Thu, 10 Apr 2003 15:53:25 +0200
Message-ID: <3E957755.4050405@cs.vu.nl>
To: WebOnt WG <www-webont-wg@w3.org>

Yuzhong Qu commented:

 > The OWL Reference says:
 > [   1.1 Purpose of this document
 >     ...
 >     The normative reference on the *PRECISE SYNTAX* of the OWL 
language constructs can be found in the OWL Semantics and Abstract 
Syntax document.
 >     ...
 >     1.3 OWL syntax
 >     An OWL ontology is encoded and written as an RDF graph, which is 
in turn a set of RDF triples. As with any RDF graph, an OWL ontology 
graph can be written in many different syntactic forms (as described in 
the RDF/XML Syntax Specification (Revised) ...).
 >     ...
 >     1.7 Appendices to this document
 >     Appendix B contains an RDF schema for the OWL language 
constructs. ....This schema provides the *BASIS* for the RDF/XML syntax 
of OWL.
 >     ...
 > ]
 > The OWL S&AS says:
 > [2. Abstract Syntax
 >     (In the first paragraph.)
 >     ....The syntax used here is rather *Informal*, ...
 > ]
 > The OWL Overview says:
 > [1.1 Document Roadmap
 >     ...
 >     The OWL Semantics and Abstract Syntax document is the *Final and 
Formally* stated normative definition of the language.
 > ]
 > It seems to me that:
 > 1) There is no PRECISE SYNTAX  of OWL.
 > 2) The XML encoding of an OWL ontology is based on RDF/XML Syntax as 
well as RDF schema for OWL (Appendix B).
 > 3) Without the PRECISE SYNTAX of OWL, where does the *Final and 
Formally* stated normative definition of the language come from?
 > I think that the specs should give a consistent and explicit stating 
on this issue.

I suggest we discuss this at the telecon.
I'm not sure about the response. It is true that Appendix B is the basis
for the RDF/XML syntax, so dropping its normative status has some
drawbacks. From S&AS the RDF/XML syntax can only be indireclty derived.

A proposal could be to restore the normative status of the Appendix B.

 > 2. The domain and range of owl:equivalentClass in OWL Lite.
 > [    3.2.2 owl:equivalentClass
 >     ...
 >     NOTE: OWL DL does not put any constraints on the types of class 
descriptions that can be used as domain and range values of an 
owl:equivalentClass statement. In OWL Lite **only class identifiers and 
property restrictions** are allowed as domain and range values. (?)
 >     8.3 OWL Lite
 >     ...
 >     the subject of owl:equivalentClass triples be named classes and 
the object of owl:equivalentClass triples be named classes, 
restrictions, or subjects of owl:intersectionOf triples (?);
 >     ...
 > ]
 > 1) According to S&AS, the domain of owl:equivalentClass must be just 
 > 2) As to the range of owl:equivalentClass, class identifiers and 
property restrictions are certainly allowed as range values. But how 
about others allowed as range values? What's "the subjects of 
owl:intersectionOf triples" mentioned in section 8.3?
 > It seems most likely to be anonymous classes defined as the 
conjunctions of class identifiers and property restrictions.
 > It (The domain and range of owl:equivalentClass in OWL Lite) should 
be explicitly and consistently specified.

I suggest we discuss this at the telecon.

We changed this at a late point, but I cannot find the record. I seem to 
remember we opted for allowing only class IDs in equivalent classes for 
OWL Lite. This is also what the OWL Lite class axioms in S&AS state.

Proposal: to make editorial changes to Ref (3.2 and 8.3) and S&AS such 
that only class identifiers are allowed for equivalent classes in OWL Lite.

 > 3. RDF schema for OWL (Appendix B)
 > 1) "rdf:resource" is a typo error as I mentioned before. It should be 

"rdf:resource is written with a lowercase in constructions like:

<rdf:Property rdf:ID="backwardCompatibleWith">
   <rdfs:domain rdf:resource="#Ontology"/>
   <rdfs:range rdf:resource="#Ontology"/>

It is in the RDF namespace, so it should really be "rdf:"
 > 2) The definition of owl:Thing and owl:Nothing
 > <Class rdf:ID="Thing">>   <rdfs:label>Thing</rdfs:label>>   <unionOf 
rdf:parseType="Collection">>     <Class rdf:about="#Nothing"/>> 
<Class>>       <complementOf rdf:resource="#Nothing"/>>     </Class>> 
</unionOf>> </Class>>
 > <Class rdf:ID="Nothing">>   <rdfs:label>Nothing</rdfs:label>> 
<complementOf rdf:resource="#Thing"/>> </Class>>
 > I suggest the axiom for owl:Thing be simplified as follows:
 > <Class rdf:ID="Thing">>   <rdfs:label>Thing</rdfs:label>> </Class>>
 > Is there any lose of meaning?

We think so. The class axiom for owl:Thing defines its class extension 
to be the extension of owl:Nothing plus its complement, which means all 
individuals in the universe of discourse. owl:Nothing is its complement, 
so its class extension is the empty set.

 > I note owl:Nothing is not included in OWL Lite. [A note in section 
3.1 Class descriptions].
 > Including owl:Nothing in OWL Lite will bring any harmness to OWL Lite ?

This is the same issue as already proposed for WG discussion by Peter.

 > 4. The rdfs:range of owl:imports
 > In Appendix C (OWL Quick Reference), the rdfs:range of owl:imports is 
missing. It should be owl:Ontology (according to Appendix B).

Thanks for spotting this. We will make the appopriate editorial change 
to Appendix C.

 > Yuzhong Qu

NOTE: new affiliation per April 1, 2003

Free University Amsterdam, Computer Science
De Boelelaan 1018a, 1081 HV Amsterdam, The Netherlands
Tel: +31 20 444 7739/7718
E-mail: schreiber@cs.vu.nl
Home page: http://www.cs.vu.nl/~guus/ [under construction]
Received on Thursday, 10 April 2003 09:53:26 UTC

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