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RE: Review of RDF Based semantics [2nd]

From: Michael Schneider <schneid@fzi.de>
Date: Wed, 8 Apr 2009 17:45:47 +0200
Message-ID: <0EF30CAA69519C4CB91D01481AEA06A00125F663@judith.fzi.de>
To: "Zhe Wu" <alan.wu@oracle.com>
Cc: <public-owl-wg@w3.org>
Hi Zhe,

many thanks for your review! See my answers to all your comments below.

>-----Original Message-----
>From: public-owl-wg-request@w3.org [mailto:public-owl-wg-request@w3.org]
>On Behalf Of Zhe Wu
>Sent: Tuesday, April 07, 2009 7:58 PM
>To: alan.wu >> "Wu,Zhe"; OWL Working Group WG
>Subject: Review of RDF Based semantics [2nd]
>
>Hi,
>
>First of all, I'd like to say that it is a great document. It's very
>clearly written and I enjoyed reading it, again!

That's great, thank you! :-)

>A few minor comments as follows.
>- Section 2.1 talks about generalized RDF triples. I am wondering
>whether it is useful
>   to restrict the use of literal values as predicates.

The text currently states that the RDF-Based Semantics *MAY* be applied 
to graphs with generalized RDF triples, and generalized RDF triples
may contain literals in predicate position. This does not mean that
implementations must support such extended RDF graphs, but they will 
not lead to problems *if* implementations support them.

Technically, there is no problem with having literals in predicate 
position, since they denote individuals under the RDF-Based Semantics, 
and, as it is true for all individuals, these data individuals can 
be properties as well. 

Whether having literals in predicate position is useful or not 
will certainly depend on the respective application, but such 
questions are outside the scope of this specification document.

However, the strong relaxation may become relevant to other standards. 
For example, the same notion of "generalized RDF" is assumed by the 
following important work-in-progress specifications that have a 
dependency on the OWL 2 RDF-Based Semantics:
 
 * OWL 2 RL (see 2nd paragraph of §4.3 in the Profiles Editor's Draft at
[1a])
 * RIF+OWL (see Section 3.1.1 of the current Editor's Draft at [1b])

So I do not intend to do a change.

[1a]
<http://www.w3.org/2007/OWL/wiki/index.php?title=Profiles&oldid=21421#Reason
ing_in_OWL_2_RL_and_RDF_Graphs_using_Rules>
[1b]
<http://www.w3.org/2005/rules/wiki/index.php?title=SWC&oldid=7792#RDF_Vocabu
laries_and_Graphs>

>- Section 4.2, I am not exactly clear about 'otherwise IL("s"^^u) is not
>in LV'
>  Does it mean that "2.01"^^xsd:integer gets treated like an IRI in OWL
>2 FULL?

If you mean by "treated like an IRI" that the denotation of the 
literal "2.01"^^xsd:integer is an instance of IR \ LV, 
then you are actually right.

This is in accordance with Section 5.1 of the RDF Semantics spec [2a]
(see the third semantic condition there). In addition, 
Section 5.2 of the original OWL 1 RDF-Compatible Semantics [2b]
states that "L_I is a mapping from typed literals in V 
to their denotations in R_I." (rather than LV_I). I have also
clarified in personal conversations with former members of the 
RDF Core WG that this is /not/ a bug.

The cited text in the RDF-Based Semantics document is only a summary
of those parts in the RDF Semantics specification that are actually
relevant for the definition of an OWL 2 Full interpretation (and
certain other aspects of the RDF-Based Semantics). So I consider
a change of these parts of the RDF Semantics to be out of scope 
for OWL and rather in the responsibility of a future 
RDF working group.

So I do not intend to do a change.

[2a] <http://www.w3.org/TR/rdf-mt/#defDinterp>
[2b] <http://www.w3.org/TR/owl-semantics/rdfs.html#5.2>

>- Section 5, in the paragraph starts with Unscoped variables. "x"
>denotes an arbitrary individual in the universe.
>  I am wondering if "element" is a better word than "individual" in this
>context (mainly
>  to avoid confusion with owl individual)

The term "individual" is just what is meant here. The text sais:

[[
Unscoped variables: If no scope is explicitly given for a variable "x",
[...]
then x is unconstrained, which means that x in IR, 
i.e. "x" denotes an arbitrary individual in the universe
]]

The idea behind this convention is that if nothing specific is said about
a variable x, then x stands for an individual. The reason is that all
elements of the domain (IR) of an OWL 2 Full interpretation are individuals.

So I do not intend to change this.

>- In Section 5, does it make sense to add a semantic condition saying
>that literal values
>  cannot be used to denote a class? It is hard for me to see the meaning
>of "x rdf:type 3.1415"

This is similar to the discussion about literals in predicate position
above.
A literal is just a name for a certain data value, which is an individual
under the RDF-Based Semantics. And every individual can have a class
extension,
hence can be a class. 

Further, this is not really a discussion related to specific aspects of 
the OWL 2 RDF-Based Semantics, but already holds for the semantics of RDFS, 
and should therefore be treated by an RDF working group. Note, however, that
what you want to disallow is currently perfectly allowed (consistent) in
RDFS.

Apart from this, I would consider this a "big change". First, the semantic 
condition would be of a form that is different from all other semantic 
conditions in OWL 2 Full (and OWL 1 Full), because it would have to talk 
about vocabulary elements instead of domain elements. Second, I would expect
that such a semantic condition may have non-obvious side effects on the
semantics, and I would need to very carefully check. Given that we plan to 
vote the document into LC *today*, there is no time anymore for such deep 
analysis.

So I am not going to do a change.

>- In Section 6, second last paragraph, "IEXT(I(owl:topObjectProperty)) =
>IR x IR"...
>  for which there are no corresponding domain and range axiomatic
>triples.
>  Why?

This is a reasonable question! The text was even confusing, as I see now. 
I have changed the text, please check!

DIFF:
<http://www.w3.org/2007/OWL/wiki/index.php?title=RDF-Based_Semantics&diff=21
611&oldid=21279>

>- Section 7 gives a very interesting example on DL entailment is not
>Full entailment and how to
>   fix it through syntactic changes. It is very useful without a doubt.
>
>  I am wondering that in addition, can we describe, at a high level,
>what can be meaningfully modeled/expressed
>  using OWL 2 FULL but not with OWL 2 DL. After all, users of OWL 2 FULL
>care more about
>  using the additional expressivity than aligning inference with DL.

Section 7 is the successor to that part of the original OWL 1 RDF-Compatible

Semantics document, which covers the correspondence theorem [3a] and its 
proof [3b]. Its purpose is to show that the two semantics of OWL 2 are 
strongly aligned, as this was an overall design goal of both OWL 1 
and OWL 2. What "strongly aligned" means is expressed by the actual 
correspondence theorem in Section 7.2. The elaborate example in Section 7.1 
shows that the two semantics are not identical, but motivates the actual 
form of the correspondence theorem and its proof in Section 7.3. 
To summarize, there is some justification that the content of Section 7 
exists in our OWL 2 specification, regardless of whether OWL 2 Full 
users are concerned about alignment with OWL 2 DL or not.

On the other hand, in my eyes, questions concerning meaningful 
OWL 2 Full modeling should be treated by research papers and text 
books, and not by the formal specification of the language itself. And 
it is far from clear to me how such a text should look like. And 
given that we vote on LC publication today, it will not be 
possible anymore to create such a text.

So I am not intending to do a change.

[3a] <http://www.w3.org/TR/owl-semantics/rdfs.html#5.4>
[3b] <http://www.w3.org/TR/owl-semantics/proofs.html>

>- The Proof of the Balancing Lemma. It seems that the algorithm
>described should terminate. Can we
>   state it explicitly?

Ok. Can you please have a look whether my changes are to your satisfaction?

DIFF:
<http://www.w3.org/2007/OWL/wiki/index.php?title=RDF-Based_Semantics&diff=21
618&oldid=21611>

>- Table 8.2, if C in IC, then exists z in IR s.t. <z, c> in
>IEXT(I(owl:complementOf))
>  Should it be "z in IC" instead? I am asking because it seems to me
>that this "z" can participate
>  other comprehension conditions including owl:intersectionOf.

No, "z in IR" is intentional and sufficient. That the corresponding 
individual is a class follows from Table 5.2, by the entry
about "owl:complementOf, after the comprehension condition has "fired".

Note that "z" is an existential variable, bound to the existential 
quantifier belonging to the respective comprehension condition. 
So it is, formally, impossible that the same "z" can participate in two
different comprehension conditions.

What /may/ happen is that the individual, for which existence is ensured
by one "z" may be the same individual, for which existence is ensured
by another "z". But this is not a problem.
 
So I am not intending to do a change.

>Some editorial changes:
>- "to some extend" ==> "to some extent"

Thanks! Fixed two times.

>- "s sequence of ..." ==> "s is a sequence of ..."

This form was deliberately chosen for brevity reasons. 
Please see the "Conventions part" of Section 5. 
(No change intended.)

>- "with other words" ==> "In other words"

Yes, thanks!

>- In the paragraph before Section 3, "how to apply these components in
>OWL 2 ..."
>  ==> "how to design OWL 2 ..."

I think this would change the intended sense of this sentence, 
or may at least lead to some confusion, since the discussion
in that section is not about ontology design. 
So I am not going to change this.
 
>- Second paragraph in Section 3.4, "in order to only refer to ..."
>  ==> "To refer to ..."

Thanks!

>- Section 5.6, "be applied to some given individual"
> ==> "be applied to a given individual"

Ok. Two fixes.

>- the paragraph before Section 6.2, "and no complex class ... do appear
>there in particular"
>==> "and no complex class ... can appear there."

Ok. 

>- Resolution of Reason 1 (Annotation): ... has been removed, i.e. there
>is no ...
>==> ... has been removed because there is no ...

No, it's really meant this way. The second part of the sentence 
is *not* a reason for the first part.

>- Section 8, second paragraph. "a core obstacle ... were RDF encodings
>..."
>==> "a core obstacle ... was RDF encodings ..."

Ok. 
(I guess that there are many bugs of this sort, 
but I would need to check the whole document.)

DIFF for all the editorial changes:
<http://www.w3.org/2007/OWL/wiki/index.php?title=RDF-Based_Semantics&diff=21
629&oldid=21618>

>Cheers,
>
>Zhe

Cheers and thanks,
Michael

--
Dipl.-Inform. Michael Schneider
Research Scientist, Dept. Information Process Engineering (IPE)
Tel  : +49-721-9654-726
Fax  : +49-721-9654-727
Email: michael.schneider@fzi.de
WWW  : http://www.fzi.de/michael.schneider
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Received on Wednesday, 8 April 2009 15:46:32 GMT

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