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Minutes of the beer session

From: Jeremy Carroll <jjc@hpl.hp.com>
Date: Mon, 26 May 2003 14:04:11 +0300
To: www-webont-wg@w3.org
Message-Id: <200305261404.11545.jjc@hpl.hp.com>


A small quantity of beer was drunk at Jim's expense by Peter and myself on 
Friday evening.

We considered the following 7 issues:

A) intersectionOf semantics

B). B1, B2 in OWL DL Syntax

C) rdf:LIst decision by RDF Core

D) OWL DL syntax NP complete

E) unnamed individuals

F) ambiguity

G) OntologyProperty

An eighth issue of articulating the reverse of the mapping rules was 
considered somewhat more casually, with at least a starting point for further 
discussion within the group.

============

Summary:

Of these A,D,E,F,G have agreements between Peter and myself ready to propose 
(A requires a significant amount of work which could be done before or after 
a WG resolution). B and C we agreed in principle the proposals but peter 
wished to review proof sketches that I need to circulate. Wtih C we also 
agreed a fall-back if the proof-sketch was inadequate.

Details:

A) intersectionOf semantics
Raised:
http://lists.w3.org/Archives/Public/www-webont-wg/2003May/0116.html

Peter and I propose solution 2/ for the reasons articulated in
http://lists.w3.org/Archives/Public/www-webont-wg/2003May/0133.html


[[
*) too many examples in the guide would either be in OWL Full or would have
 to change significantly with the proposed changes to the mapping rules.

*) The proposed changes to the OWL Full semantics makes various intuitive
entailments hold (the ones that are currently in doubt)
]]

This changes oen if-then in the rdfs semantics to an iff, and necessitates 
work on the correspondence proof.

B). B1, B2 in OWL DL Syntax

We agreed this was desirable:
ACTION: jjc Send proof sketch of extension to correspondence theorem with B1 
B2 under new intersectionOf semantics.
ACTION: pfps review proof sketch of correspondence with B1 B2 
(this replaces old action on pfps to work on this proof, withdrawn under (A) 
above).

C) rdf:LIst decision by RDF Core
We agreed in principle that:
C.1) we could handle this by simply making the rdf:LIst triple optional in the 
mapping rules
C.2) it is better to go further and make the type triple optional on all blank 
nodes in OWL DL

ACTION: jjc send proof sketch that nothing breaks without type triples for 
blank nodes
ACTION: pfps review proof sketch of blank nodes without type triples

Flow chart:
  Proof works  ==yes==> propose C.2)
                    ==no==> propose C.1)

We note that the desciption of OWL Lite and OWL DL  in english would become 
shorter with this change.

D) OWL DL syntax NP complete

Included in comments
http://lists.w3.org/Archives/Public/public-webont-comments/2003May/0083.html
http://lists.w3.org/Archives/Public/public-webont-comments/2003May/0052.html
owlsas-rdfcore-np-complete

Original message:
http://lists.w3.org/Archives/Public/www-webont-wg/2003Apr/0003.html

We agreed on solution 2 at the end of the original message.

change mapping rule to
EquivalentClasses( d1, .... dn )
==>
 T(dj) owl:equivalentClass T(di) .   For all i,j s.t {i,j} in G, an arbitrary 
connected graph over {1, 2, ... n }

(Exact wording is editorial)
We agreed that the expected reader of S&AS is also expected to understand 
terms from graph theory without any explanation, i.e. this rule will be 
opaque to the non-specialist.

E) unnamed individuals
http://lists.w3.org/Archives/Public/public-webont-comments/2003May/0052.html
owlsas-rdfcore-bnodes-restrictions

Peter and I propose we postpone this issue.
Rationale:
- the amount of work to do otherwise is large, and is not guaranteed to 
succeed.
- however the point is a good one and should be reviewed once the community 
has more experience.

(Personal note: my reading is that much of the approach taken in S&AS part 3 
would need substantial reworking to address this)

F) ambiguity
http://lists.w3.org/Archives/Public/public-webont-comments/2003May/0055.html
[[
"Non-deterministic"?  Scary, if the goal is well-defined
semantics.
]]
http://lists.w3.org/Archives/Public/public-webont-comments/2003May/0059.html
[[
3) The optional and non-deterministic mappings to/from triples are a
   bad idea that are likely to cause interoperability problems and
   make the mappings harder.  I urge you to consider removing such
   non-determinism.
]]

http://lists.w3.org/Archives/Public/public-webont-comments/2003May/0083.html
[[
D: allowing blank restriction nodes to have class owl:Class
...
**Comment 3**
* *D* is clunky and we ask the group to reconsider both optional triples in
mapping rules such as:

restriction(ID maxCardinality(max))
==>
_:x rdf:type owl:Restriction .
_:x rdf:type owl:Class . [opt]
_:x rdf:type rdfs:Class . [opt]
_:x owl:onProperty T(ID) .
_:x owl:maxCardinality "max"^^xsd:nonNegativeInteger .

(the owl:Class optional triple is more problematic than the rdfs:Class, 
since it makes the rule on requiring explicit type for all nodes more 
complicated.
owl:Class is a possible explicit type for classID and description nodes, 
but not for restriction nodes). We suggest removing the optional triples 
from this rule, and other similar rules.
]]


The current text says:
"These non-determinisms allow the generation of more RDF Graphs. "

Peter and I agreed that these comments should be handled by editorial 
expansions on that sentence to better motivate the non-determinstic aspects 
of the rules.
(Note, my agreement is dependent on issue C where we make the type triples 
optional for bnodes, which addresses the more detailed comments.)
peter to propose any textual changes.

G) OntologyProperty

Again in
http://lists.w3.org/Archives/Public/public-webont-comments/2003May/0083.html
[[
**Comment 4**
A further clunkiness was with owl:OntologyProperty.
Triples such as
owl:priorVersion rdf:type owl:OntologyProperty .
are permitted by the grammar iff owl:priorVersion is used somewhere else.
We have correctly implemented this, but it is surprising.
]]

peter and I propose that we add an ontologyproperty directive which mirrors 
the annotationproperty directive in the abstract syntax, with appropriate 
mapping rules. This will permit user defined ontology properties and 
annotations on ontology properties.
Looking at the issue list this does not seeem to contradict the letter of 
issues 5.14 Ontology versioning and 4.4 Extra-logical feature set.
But it does seem worth noting those.


====

We also discussed the issue of an alternative articulation of the mapping 
rules to go in the other direction. We were not convinced that these comments 
could be addressed with a relatively small amount of work. A possible 
approach is to sidestep them by pointing to two implementations of an OWL 
Syntax Checker (which we already have, modulo the last details). This is the 
initial proposal from the beer session.

The possible additional text that we have is:
- the description of RDF graphs as triples
- the related proof (incomplete)

The related proof does address the reverse mapping problem, but is unfinished 
and difficult to read. It does include tables (which I don't think I ever got 
round to generating) of when you see this triple then use this abstact syntax 
construct. The amount of work needed to get it finished is nontrivial.

The description of RDF graphs as triples could be updated failry quickly, but 
only permits the building of a recognizer (an OWL Syntax Checker) and not a 
tool which outputs the abstract syntax tree.



Jeremy








 
Received on Monday, 26 May 2003 08:04:08 GMT

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