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Re: RDF Semantics: two issues, connected to OWL

From: <herman.ter.horst@philips.com>
Date: Fri, 12 Dec 2003 20:58:07 +0100
To: pat hayes <phayes@ihmc.us>
Cc: connolly@w3.org, sandro@w3.org, www-rdf-comments@w3.org
Message-ID: <OF2170019C.3E0B89FB-ONC1256DFA.005C4E55-C1256DFA.006DBCF8@diamond.philips.com>

[...]

>>
>>Issue 1- It seems that two changes made to RDF Semantics
>>during LC2 have not yet been incorporated completely
>>in the definition of D-interpretations.
>
>As already remarked in earlier messages: that I regard as a typo, and 
>plan to get it changed before final publication , if the process will 
>allow it.

Without going into the meaning of the word typo, I would only like
to note for clarity that I added two changes in my previous message, 
to the change about which you remarked in [5] that you regard it as a
typo.


>
>>Issue 2- There remains a problem with the combination of the two
>>documents  OWL S&AS (Semantics and Abstract Syntax)
>>and  RDF Semantics, for which the optimal solution
>>(in fact, any complete solution) seems to require changes to
>>RDF Semantics [2].
>
>You may well be right that the most elegant solution would be based 
>on the technique you describe below. However, I do not agree that 
>this is optimal, for a number of reasons. First, the fact that RDF 
>reasoning  - even without datatyping - includes an accommodation to 
>rdf:XMLLiteral  has been part of the RDF design for some time now, 
>and the documents have drawn explicit attention to it.  The language 
>was designed this way deliberately, as a result of WG decisions made 
>partly in response to other comments; so to change this at this stage 
>would be a major re-design.  Webont had adequate time to comment on 
>this aspect of the design if they had wished to, and they did not. 
>So if this fact causes some difficulty for Webont, then at this very 
>late stage in the process I feel that it is up to Webont to correct 
>the matter by making appropriate changes to the OWL documentation. 
>However, read on.
>
>>The problem I refer to in issue 2 is that the OWL design
>>includes the possibility to do without the semantic conditions
>>of XMLLiteral.
>>This aspect of the OWL semantics design is not reflected in S&AS,
>>and came to my attention only a few days ago [3] [4].
>
>I am somewhat confused by this. You seem to be saying that the OWL 
>documentation agrees with the RDF documentation, but that both of 
>them are at odds with an undocumented (?) "OWL design". To my mind 
>there is no problem here. The actual OWL design is that which is 
>described in the documents. If someone misunderstands these documents 
>then the appropriate course of action is for them to improve their 
>understanding.

I continue the discussion of issue 2 in the webont mailing list.
Here I only make the following remark, which I hope dissolves
your confusion:
The only way in which this "OWL design" surfaces in the documents
is in the test document: there are OWL tests that do not require
the support of XMLLiteral.  S&AS, being a semantic extension
of RDFS, does require the support of XMLLiteral.

>
>>This possibility is not realized by S&AS because OWL is a
>>semantic extension of RDFS, and each rdfs-interpretation
>>satisfies the XMLLiteral conditions.
>
>Quite. So this 'possibility' is in fact not a possibility.
>
>I do not feel that any further action needs to be taken on your issue 2.
>
>Pat
>
>>
>>I discuss a possibility to solve both issues.
>>This possibility leads in addition, without much additional
>>cost, to a possible, interesting extension of the RDF Semantics
>>document, namely a sound and complete proof theory for reasoning
>>with datatypes: an "RDFS-D-entailment lemma", generalizing
>>the RDFS-entailment lemma to a large class of datatype maps.
>>
>>===
>>
>>Details:
>>
>>The two changes I refer to in issue 1 above are the omission of
>>the terminology "from V" and the change not to require all
>>possible literal values in LV.  In line 1 of the definition
>>of D-interpretations the conclusion aaa in V would need
>>to be added before I(aaa)=x, and line 2 of this definition
>>could be generalized to include not all values in LV.
>>The point of such a generalization would be - in line with
>>the spirit of the document - that RDF aims at generality,
>>so that semantic extensions have most freedom,
>>and RDF is a flexible basis for the Semantic Web.
>>(It was noted earlier that line 3 of this definition has also
>>not been completely corrected, see [5].)
>>
>>The following definition of D-interpretations is formulated
>>as a strict extension from the XMLLiteral conditions:
>>
>>Given a datatype map D and a vocabulary V, a D-interpretation
>>of V is an rdfs-interpretation of V such that for all
>><a,d> in D we have:
>>- a in V and I(a) = d
>>- I satisfies the triples
>>     a type Datatype
>>     a subClassOf Literal
>>- if l=s^^a is a typed literal in V and s in L(d),
>>   then IL(l)=L2V(d)(s) in LV  and  <IL(l),d> in IEXT(I(type))
>>- if l=s^^a is a typed literal in V and s not in L(d),
>>   then IL(l) not in LV  and  <IL(l),d> not in IEXT(I(type)
>>
>>This definition of D-interpretations would solve issue 1 above.
>>
>>If this change would be made, then the XMLLiteral conditions
>>could be removed from Section 3 of RDF Semantics, and
>>incorporated by including XMLLiteral in a datatype map.
>>When the RDF Semantics document would be changed so that XMLLiteral is
>>not necessarily present in every datatype map, then issue 2 would
>>be solved as well, and the inconsistency between the documents
>>OWL S&AS and RDF Semantics that was noted would be gone.
>>
>>If these changes would be made, then the RDF and RDFS entailment
>>lemmas would need change as well, since they include XMLLiteral.
>>Below I show that an attractive generalization of the
>>RDFS entailment lemma could be obtained, extending the result
>>to include a large class of datatype maps.
>>
>>=
>>
>>Given a datatype map  D, consider the following "D-entailment rules":
>>for each pair <a,d> in D, in order to replace rule rdf2:
>>rule "rdf-a":
>>   if E contains  uuu bbb lll, where lll = sss^^a is a well-typed 
literal,
>>   then add  _:nnn type a, where _:nnn identifies a blank node
>>   allocated to lll by rule lg.
>>
>>Define a "D-clash" to be a triple
>>    b type Literal
>>where b is a blank node allocated by rule lg to a literal s^^a, where
>><a,d> is in the datatype map D for some d.
>>
>>The definitions suggested above lead to the following generalization
>>of the RDFS entailment lemma:
>>Suppose that S is a set of RDF graphs, G an RDF graph, and D
>>a datatype map with disjoint value spaces and injective
>>lexical-to-value mappings.
>>Then S D-entails G iff there is a graph H that can be derived
>>from S combined with axiomatic triples, by application of the
>>literal generalization rule, rdf1, and RDFS and D-entailment
>>rules, and that either simply entails G or contains a D-clash.
>>
>>The injectivity assumption can often be handled in practice with
>>a suitable canonicalization operation.
>>Assuming here the presence of rule rdfs14 :-) [1], this result
>>holds: the details of the proof can be worked out.
>>The assumptions on D are used in connection with the definition
>>of the surrogate function sur.
>>
>>
>>
>>
>>Herman ter Horst
>>
>>[1] http://lists.w3.org/Archives/Public/www-rdf-comments/2003OctDec/0205.html
>>[2] http://lists.w3.org/Archives/Public/www-webont-wg/2003Dec/0042.html
>>[3] http://lists.w3.org/Archives/Public/www-webont-wg/2003Dec/0034.html
>>[4] http://lists.w3.org/Archives/Public/www-webont-wg/2003Dec/0035.html
>>[5] http://lists.w3.org/Archives/Public/www-rdf-comments/2003OctDec/0218.html
>
>
>-- 

Herman
Received on Friday, 12 December 2003 14:58:44 GMT

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