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Re: [ontolog-forum] Interpreting OWL

To: "ian@xxxxxxxxxxxxxxxx" <ian@xxxxxxxxxxxxxxxx>, "[ontolog-forum]" <ontolog-forum@xxxxxxxxxxxxxxxx>
From: Ed Barkmeyer <edbark@xxxxxxxx>
Date: Mon, 18 Oct 2010 13:23:25 -0400
Message-id: <4CBC828D.3030603@xxxxxxxx>
Ian,    (01)

I tend to agree with your comments.  Notes embedded.    (02)

Ian Bailey wrote:
> Hi Sean,
> I would be tempted to leave OWL out of the equation altogether for 
> EXPRESS, and use RDF/RDFS. The most used OWL representation is the RDF 
> one, so I think this makes sense for EXPRESS too.
>    (03)

This is also what David Price suggested.  The question that has not been 
answered is:  Why OWL?  What is it that Sean and gang want to get from 
OWL, other than academic acceptance of their work (John Sowa's codicil 
being relevant in that regard)?    (04)

>  I think you could do a pretty good job of modelling the EXPRESS 
> meta-model in RDF(S). It would avoid a lot of the baggage that comes 
> with OWL (I can hear the DL nuts lighting their torches as I type) and 
> would be using a similar approach to the RDF form of OWL. This would 
> then make the comparison you mention a bit more straightforward.
>    (05)

I pointed to the EXPRESS MM specification in my previous email.  OMG is 
currently near standardization of an RDF(S) form of the MOF, which gives 
the metamodel a standard RDF form (for what that may be worth).    (06)

>  There are a few oddball aspects in EXPRESS such as SELECTs and 
> SUPERTYPE OF clauses that, although they’ll map, won’t turn out quite 
> as the STEP community would expect them to.
>    (07)

Interesting.  I think SUPERTYPE (and SUBTYPE_CONSTRAINT its preferred 
EXPRESSv2 form) should map very readily to useful OWL constructs.  The 
one thing DLs do well is describing relationships among Sets.  If that 
isn't what the STEP community expects, they don't know what their model 
means.    (08)

Similarly, a SELECT type is just an after-the-fact supertype, typically 
created in order to define the domain or range of some property or 
properties.  This is also something that is easy to do in OWL/DL, with 
exactly the EXPRESS interpretation.  What OWL doesn't have is the 
EXPRESS computational overload in the assumption that an instance can be 
asked what all types it instantiates.  That is a purely programming 
language idea.  Logically it makes no sense at all.  It IS meaningful to 
ask whether a given thing is an instance of a given type -- it is 
something you ask all the time in SPARQL.   (I lost this modeling battle 
with the EXPRESS Committee in SC4/WG5 in 1989, thus creating 20 years of 
bad modeling practice.)    (09)

>   All you need to do then is persuade the OMG to do the same thing – 
> though there is the added complication of UML being wedded to MOF.
>    (010)

I missed some connection here.  I think what Ian is saying is that there 
should be a standard RDF form of the OMG EXPRESS metamodel.  See above.     (011)

The limitations of MOF have had only minor consequences for the EXPRESS 
Metamodel.  Bear in mind that the metamodel defines, for example, 
SelectType as a meta-class of things that may appear in an EXPRESS 
schema.  It does not attempt to assign any UML meaning to those things; 
it assigns the SelectType (EXPRESS SELECT) meaning to those things.  We 
modeled the EXPRESS language.  Mapping the concepts of that metamodel to 
concepts of the UML metamodel or concepts of the OWL metamodel is a 
separate task.  All the EXPRESS Metamodel provides is the formal 
meta-objects that such a mapping can manipulate.  Where there are no 
exact equivalents, the mapped image of an EXPRESS model element will 
suffer from losses of some semantics in the domain element and/or gains 
of unintended semantics in the range element.    (012)

> An RDFS representation of MOF might even give us a replacement for XMI 
> that actually works, and that can’t be a bad thing.
>    (013)

That is one of the motivations of the MOF2RDF proposers, although it is 
impolitic to state it formally in an OMG RFP.  The real argument is that 
XMI is a foreign language to almost all programmers, whereas RDF has a 
significant presence in the larger "modeling community" that includes 
XMI's only constituency.    (014)

With respect to XMI, OMG finally decided that its reputation was at 
stake and "locked the UML vendors in a room" -- the Model Interchange 
Committee -- to agree on a standard interpretation of UMLv2 into an 
interchange form.  We are finally, after only 5 years, beginning to see 
XMI interchange work among UML tools.    (015)

> Cheers
> --
> Ian
> *From:* ontolog-forum-bounces@xxxxxxxxxxxxxxxx 
> [mailto:ontolog-forum-bounces@xxxxxxxxxxxxxxxx] *On Behalf Of *sean barker
> *Sent:* 17 October 2010 12:17
> *To:* ontolog-forum@xxxxxxxxxxxxxxxx
> *Subject:* [ontolog-forum] Interpreting OWL
> Apologies for asking another question before I have finished 
> responding to the last, but a question arose at the last STEP meeting 
> which has some tricky implications.
> The statement was made that anything that could be written in EXPRESS 
> could be written in OWL. However, some of the constructs in EXPRESS, 
> particularly those concerning the cardinality and structure of 
> relationships are not directly obviously expressible in OWL, such as 
> the distinction between a bag and a set. However, it should be 
> possible to create a first order interpretation of OWL such that an 
> EXPRESS relationship is a subtype of 'thing', and the relationship 
> constraints are then OWL properties. EXPRESS Entity and Type also 
> become subtypes of 'thing'
> This then allows one to construct a second order interpretation by 
> suptyping Entity, Relationship and Type as STEP generic entities, such 
> as Product, Version, View, Property, Property-Representation, 
> Representation-Presentation etc. That is, EXPRESS entities provide an 
> upper level ontology for STEP in OWL.
> One can then create a third order interpretation, as is done in the 
> STEP Application Protocols, in which the STEP generic entities are 
> interpreted in the context of a business process, so that Product is 
> either a product (AP 203) a part (AP 214) or a technical data package 
> (AP 232).
> Two questions arise. Firstly, one could also describe other modelling 
> languages such as UML and IDEF1X as first order interpretations of OWL 
> (in the sense above) (and even of OWL itself). Could one then compare 
> the expressive power of such formalisation by creating a lattice of 
> modelling languages? (This would also expose ambiguities in the 
> languages).
> Secondly, would the second and third order interpretations be 
> compatible with anybody else's use of OWL? For example, whether a 
> particular (EXPRESS) property is a property of a product is contingent 
> on the Version (an implied temporal commitment) and the View.
>    (016)

Edward J. Barkmeyer                        Email: edbark@xxxxxxxx
National Institute of Standards & Technology
Manufacturing Systems Integration Division
100 Bureau Drive, Stop 8263                Tel: +1 301-975-3528
Gaithersburg, MD 20899-8263                FAX: +1 301-975-4694    (017)

"The opinions expressed above do not reflect consensus of NIST, 
 and have not been reviewed by any Government authority."    (018)

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