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Re: OWL comment - blank nodes in OWL DL

From: Jim Hendler <hendler@cs.umd.edu>
Date: Mon, 21 Jul 2003 17:15:55 -0400
Message-Id: <p05200f06bb42083a8f09@[]>
To: Dave Reynolds <der@hplb.hpl.hp.com>, public-webont-comments@w3.org

  thanks for all the comments you raised on the 
public-webont-comments@w3.org list.  This is the response to the 
remaining one:

>This is a comment about blank nodes in OWL DL on behalf of the Jena team.

>   - We would like it to be possible to name dataranges

The Web Ontology WG discussed this issue and others to do with naming 
various datatypes under the issue "datatypes" which is issue 5.8 on 
our issues list [1] and in our issue 4.3 of "structured datatypes" 
[2], which included a discussion of naming datatranges raised by Ziv 
Hellman [3]
Summarizing what can be seen in the discussions referenced there, the 
WG felt that
   (i) we should yield to RDF Core's decisions on datatyping, and
   (ii)that we shouldn't create new URIs for datatypes that might 
conflict with an eventual decision in the handling of XML Schema 
Datatypes by the XML Schema group.
  In light of these considerations, the group decided to postpone 
issue 4.3 and not add named dataranges at this time.
  The above assumes your question is specifically about complex 
datatypes and ranges (i.e. being able to say a teenager has as age 
property value between 13 and 19).  Unamed datatranges could also be 
asked about the ability to create datatypeProperties that are created 
in a manner corresponding to the creation of unnamed 
ObjecttypeProperties.  This was not identified by the WG as a 
requirement and raises some issues of a research nature as to the 
handling of these within the DL restrictions.

- It is natural in Jena to permit the java objects
   representing blank nodes which
   represent descriptions and restrictions to be
   used more than once, and hence as the object of
   more than one triple.

   A rationale for not permitting this in OWL DL
   should be given, preferably as a test case in OWL Full
   showing an OWL Full non-entailment that would hold in
   OWL DL if such triples were permitted.
   Below is an example such a test case, which we believe is an
   OWL Full entailment. If on the contrary it were a
   non-entailment, that would satisfy this comment.

The issue of the proper treatment of blank nodes is a complex one, 
and has received a great deal of discussion on the WG.  Issue 5.26 
[4] involves many subissues involving the mapping between RDF and 
OWL.  Part of that discussion involved the treatment of blank nodes 
similar to your discussion above.  The group was unable to reach 
consensus on this issue, and in fact ended up closing the issue over 
the objection of Jeremy Carroll of Hewlett-Packard, his objection can 
be seen in [5].  The WG was concerned that the handling of blank 
nodes has not yet been shown to be able to be handled in the 
correspondence proof of Appendix A of the Semantic Document [6].

- The RDF support within Jena permits users to:
   - use annonymous nodes as the object of
     more than one triple
   - have cycles of anonymous nodes
   While the syntactic restrictions between, for example,
   unnamed individuals and descriptions are understandable,
   it is not clear why OWL DL has not permitted, for example,
   an unnamed individual to be the object of more than
   one triple, or an unnamed individual to be the object
   of a triple of which it is the subject.
   Please either relax this constraint or offer a rationale.

I am pasting in here the response that Peter Patel-Schneider sent to 
another comment raising this same issue - his answer can be seen in 
full in [7]

The constraints on owl:equivalentClass triples can be articulated in terms
of Hamilton paths in the component graphs that are created by considering
only connected groups of blank nodes and named nodes that are connected to
these blank nodes.  Neverthless, this is expensive to implement.

The working group decided on 29 May 2003, as recorded in
http://lists.w3.org/Archives/Public/www-webont-wg/2003May/0402.html, to
change the mapping for owl:equivalentClass from paths to connected graphs.
This should be much easier to implement.

This change is reflected in the editor's draft of S&AS as of 30 May 2003,
which says:

S: EquivalentClasses(description1 ˆɬˆǬ descriptionn)

T(S): T(descriptioni) owl:equivalentTo T(descriptionj) .
	for all <i,j> in G where G is a set of pairs over
	{1,...,n}x{1,...,n} that if interpreted as an undirected graph
	forms a connected graph for --------

If you'd like to review this in context, you can take a look at the
editor's draft, in the the "Transformation to Triples" table at

I hope these responses will demonstrate the rationale behind the 
decisions the WG has made on these issues.  We hope you can accept 
our postponement of one, reluctance to change our response on 
another, and changes made to fix the third.
  -Jim Hendler
  for WOWG

[1] http://www.w3.org/2001/sw/WebOnt/webont-issues.html#I5.8-Datatypes
[3] http://lists.w3.org/Archives/Public/www-webont-wg/2002May/0040.html
[5] http://lists.w3.org/Archives/Public/www-webont-wg/2003Mar/0264.html

Professor James Hendler				  hendler@cs.umd.edu
Director, Semantic Web and Agent Technologies	  301-405-2696
Maryland Information and Network Dynamics Lab.	  301-405-6707 (Fax)
Univ of Maryland, College Park, MD 20742	  *** 240-277-3388 (Cell)
http://www.cs.umd.edu/users/hendler      *** NOTE CHANGED CELL NUMBER ***
Received on Monday, 21 July 2003 17:19:41 UTC

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