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.