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