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Re: POWDER formal semantics 4.3 comments

From: Stasinos Konstantopoulos <konstant@iit.demokritos.gr>
Date: Wed, 26 Nov 2008 20:33:55 +0200
To: Jonathan Rees <jar@creativecommons.org>
Cc: public-powderwg@w3.org, Alan Ruttenberg <alanruttenberg@gmail.com>, Michael Schneider <schneid@fzi.de>
Message-ID: <20081126183355.GD6466@iit.demokritos.gr>


Dear Jonathan,

thank you for reviewing the POWDER formal doc and for your helpful
comments. Responses inlined, please read on.


On Wed Nov 19 13:46:19 2008 Jonathan Rees said:

> 1. I couldn't find a specification of the particular regular expression 
> syntax used (e.g. perl5). (Obviously this would be done by reference, not 
> inclusion.) A citation in section 4.3 would be especially helpful.

This shall be taken care of, thank you for your comment.

> 2. Regarding the use of XMLLiteral in section 4.3, an OWL WG chair tells 
> me: "There's an issue in that it is not certain that XMLLiteral will be a 
> supported datatype in OWL 2 DL. It will be in OWL 2 with RDF Semantics.
> http://lists.w3.org/Archives/Public/public-owl-wg/2008Nov/0110.html
> If you think that will be a mistake, please send mail to public-owl- 
> comments. Soon."

This does not influence the formal doc which only uses XMLLiteral
at the RDF level before OWL vocabulary is interpreted. It does,
however influence POWDER as a transport layer for XML meta-data
(DR doc sect. 2.8.2) because if OWL abolishes XMLLiteral,
<owl:hasValue> will not permit XML as a value (as in, for example,
Ex 2-15, lines 36--41).

We shall contact OWL-WG, thank you for bringing this to our
attention.

> 3. Regarding "wdrs:matchesregex rdfs:domain rdfs:Resource" -- this won't 
> work in OWL DL. If you care about DL you should replace rdfs:Resource 
> with owl:Thing. I don't think anything will suffer much for doing so.

This does, in fact, work because
  wdrs:matchesregex rdf:type owl:DatatypeProperty
  wdrs:notmatchesregex rdf:type owl:DatatypeProperty
implicitly restrict the domain to owl:Thing. 

> 4. IEXT is not defined; I think you should cite the document that  
> defines it. It appears you mean the 2004 RDF Semantics recommendation 
> http://www.w3.org/TR/2004/REC-rdf-mt-20040210/ .  I see that you cite it 
> in 4.6 but it is needed in 4.3.

This shall be taken care of, thank you for your comment.

> 5. If you mean for POWDER to work with OWL-DL, have you had anyone  
> review the POWDER semantics with an eye to interaction with OWL-DL model 
> theoretic semantics? Would you consider saying something about this in 
> your document, since OWL-DL semantics differs from RDF semantics?

Several people have reviewed the document, so far not raising any
objections on this issue. Is there some particular problem you have in
mind?

> 6. "equivalence relation" has a technical meaning in mathematics and I  
> don't think it's what you mean here. I think that if you just say  
> "relation" you will convey the right thing.

We didn't expect to get this reading from anybody, especially since it
is immediately obvious that this relation is not an "equivalence
relation" in the theoretical-algebra sense. It is used in the looser sense of
the final note of Section 5.1 of "RDF Concepts" [3].


On Thu Nov 20 12:06:14 2008 Jonathan Rees said:

>
> I'm diving a bit deeper into the relation between RDF formal semantics  
> [1] and POWDER formal semantics [2], and have found another glitch.
>
> POWDER says:
>
>           o uuu is in the domain of I, with I(uuu)=x
>
> Clearly uuu is meant to be an IRI. But RDF semantics says that the  
> "domain" of an interpretation I is a set of resources (somewhat  
> confusingly, since I is also used as a function that has a domain that  
> is syntactic). I think you mean for uuu to belong to V, the "vocabulary" 
> of I, which is the domain not of I but of IS:

In this case, I is used as a function, as shown by its being
applied to uuu in "I(uuu)".

This function has a domain or universe a non-emtpy set IR,
which includes as a subset the literal vocabulary LV.

> "A set of names is referred to as a vocabulary [V]."  ... "4. A mapping 
> IS from URI references in V into (IR union IP)" ... "if E is a URI 
> reference in V then I(E) = IS(E)"
>
> I couldn't find any particular restrictions on what V might be; it could 
> be empty, or the set of IRIs, or the set of IRIs occurring in the graph, 
> or anything else. I would guess that in applying an interpretation to a 
> graph, the name (IRI) set is meant to at least contain the vocabulary of 
> the graph (the IRIs) occurring in it, but it could be limited to it.

There is also no restriction in the RDF doc (as far as I could tell)
that the set of URIrefs be disjoint from LV. (NOTE that OWL-DL
requires that owl:ThingS and LV are disjoint, but the domain of IS is
the references, not the logical entities themselves). The POWDER
extension implies that URIrefs are in LV, so that:

  if E is a URI reference in V then I(E) = IS(E)

will take us from an rdf:Literal to the resource, if the literal
happens to be a well-formed URI reference; applying IL(E) otherwise.

The semantic extension itself is needed to also be able to apply IL to
the XMLLiteral so that a regexp can be applied to the actual string.

Please comment.

> Now there are two problems with this. First, you want to talk about  
> IRIs, not URIrefs, right? That is, if the RDF graph contains the  
> relative URIref "a/b", you would prefer to match against the fully  
> resolved IRI, not the URIref, since otherwise the truth of a POWDER  
> graph would depend on choice of base IRI, which makes no sense. So you  
> need to have a story that accounts for the base URI (or other resolution 
> mechanism), or else arranges for all IRIs to be fully resolved by the 
> time they get to this point. Perhaps this is already taken care of, and 
> I'm just missing it.

It is our understanding that URIrefs are by definition absolute and
unique identifiers across any RDF document. (cf. RDF Semantics,
Sect 1.2 [4]). Since RDF semantics does not restrict how this can
be achieved, neither does the POWDER at the abstract, formal level.

At the operational level, one can easily imagine that certain
normalization and canonicalization steps are required. These are
specified in the DR and Grouping docs.

> Second, the restriction of POWDER formal semantics to the IRIs that are 
> in the vocabulary of the interpretation (= domain of IS = V) will only 
> sometimes agree with the informal semantics that you are trying to 
> capture. Suppose a graph contains assertions that the resources named by 
> a/x, a/y, and a/z (imagine now these are IRIs) are green. If these are 
> the only IRIs of the form a/* occurring in V, one could conclude, 
> according to your semantics, that all the resources in the group defined 
> by IRI pattern a/* are green. But there might be another resource, a/w, 
> that is not green, but just happens to not be mentioned in this 
> particular graph. The formal semantics would agree with the informal 
> semantics only in the case that all resources with IRIs matching a/* 
> occur in the graph.

I might be misunderstanding you here, but you seem to be introducing an
inductive step which is not present in either the operational or the
formal semantics. If a POWDER document asserts that "a/*" are green,
that is because its author makes this assertion based on their
understanding and/or inspection of the real-world domain; the
assertion holds regardless of which entities of "a/*" happen to be
represented in any given RDF document.

Once again, for absolute clarity, POWDER documents DO NOT describe
generalization or explanations processes; POWDER documents are
purely deductive models, just as OWL is. If you have gotten a
different impression, please point out the part of the text where
you feel this impression is given so that we can clarify the text.

> On the other hand, model theoretic semantics (for either RDF or OWL-DL) 
> might handle this well, since entailment is quantified over all possible 
> models, and at least one of these will include information about a/w. But 
> the fact that some of your interpretations are wrong means that you will 
> get fewer entailments than you might otherwise like. It might be worth 
> the effort some time to restrict interpretations further.

In the light of this paragraph, it seems possible that I
misunderstood your previous paragraph, and what you are saying above
is that we are asserting the *existence* of resources that match
regexps, thus loosing perfectly valid interpretations of POWDER-RDF
graphs. This is not the case since there is no POWDER/XML document
that (using the prescribed transform) will generate OWL/RDF with
existentially quantified assertions about owl:Thing instances; POWDER
docs assert universally quantified implications (subsumptions) linking
existentially-quantified nodes representing intermediate classes
(irisets and descriptorsets); such classes must exist but may very
well be empty. Furthermore the POWDER Processor semantics make it
clear that one can only query a POWDER doc about the description of a
named resource.

Best,
Stasinos


> [1] http://www.w3.org/TR/2004/REC-rdf-mt-20040210
> [2] http://www.w3.org/TR/2008/WD-powder-formal-20081114

[3] http://www.w3.org/TR/2004/REC-rdf-concepts-20040210/#section-XMLLiteral
[4] http://www.w3.org/TR/2004/REC-rdf-mt-20040210/#urisandlit
Received on Wednesday, 26 November 2008 18:34:45 GMT

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