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Re: ISSUE 5.6 - daml:imports as magic syntax

From: Peter F. Patel-Schneider <pfps@research.bell-labs.com>
Date: Mon, 09 Sep 2002 18:15:14 -0400 (EDT)
Message-Id: <20020909.181514.78820216.pfps@research.bell-labs.com>
To: heflin@cse.lehigh.edu
Cc: www-webont-wg@w3.org

A very good summary of daml/owl:imports.

From: Jeff Heflin <heflin@cse.lehigh.edu>
Subject: ISSUE 5.6 - daml:imports as magic syntax 
Date: Mon, 09 Sep 2002 17:23:46 -0400

> Problem: 
> --------
> If daml:imports is just another RDF or OWL property, then users can
> place restrictions, etc. on it. For example, consider a situation where
> there was a max cardinality of 1 on imports. If an ontology imported two
> or more ontologies, then this would effectively say that the ontologies
> are equivalent. It is probably best to avoid such situations.
> 
> Possible Solution:
> ------------------
> This seems to indicate that imports should be a "dark" property. Note
> that this isn't an issue in the Abstract Syntax, since in it, everything
> is essentially "dark." 

A treatment of imports can be done completely syntactically, by replacing
imports foo, where foo is a URI (or whatever) by the contents of the
document pointed at by foo.   This is the way I would handle it in the
abstract syntax and direct semantics.

> In Pat Hayes' recent OWL model theory, it should
> be sufficient to say that owl:imports is not an instance of owl:Object,
> owl:DatatypeValue, owl:Class, owl:ObjectProperty, or
> owl:DataTypeProperty. However, it would still be an rdfs:Property, but
> any RDF graph that uses it as a subject or object would not be
> well-formed OWL/RDF. Note, owl:imports doesn't fit into any of the
> classifications in the table in section 2 of the Hayes model theory, so
> perhaps a new classification called metaproperty or such is needed.

Even in this separated treatment, imports is problematic.  It would require
a completely new kind of semantic rule, and one that brings new kinds of
things into the semantics.  

> Semantics:
> ----------
> It is important that the semantics of owl:imports be added to any
> semantics documents. To do this, we need to be able to refer to sets of
> OWL statements (such as a web page, a database with an OWL interface,
> etc.) This could probably be called a resource, but that term is also
> used to describe RDF instances, so for lack of a better term, I will
> choose the term graph for the time being. Let graph be a function from a
> URI (URL?) to an RDF/OWL graph. Each OWL graph has a set of entailments
> that are determined by the model theory. The semantics of a statement:
> 
> A owl:imports B.
> 
> are:
> 
> if graph(B) |= X then graph(A) |= X
> 
> (Note: Here, "|=" is the OWL entailment relation)

This adds in a lot of error-prone machinery.

> One question is whether we need something more specific in the model
> theory that, for example, uses ICEXT and IEXT.

Something more is needed.  In particular, a relationship between the
current graph and its name is needed.  Otherwise how can the condition
be discharged in the conditional above?
 
> Any comments or suggestions?
> 
> Jeff

Note that the DAML+OIL axiomatization does not handle imports at all.

peter
Received on Monday, 9 September 2002 18:15:23 GMT

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