vass-01 flat layering

Danbri and I have been discussing issue:

   http://www.w3.org/2001/sw/RDFCore/20030123-issues/#vass-01

Its quite hard to get a crisp quote of the problem from the email thread 
but I think the issue is that schema does not layer the same way as, say 
UML, in that classes can be members of themselves and one can take a 
subproperty of subPropertyOf or rdf:type.

I think we need to clearly articulate the advantages of the design choice 
that has been made.

I suggest the following reason for the design choice and why we should not 
change.  I welcome comments and other suggestions:

1) RDFS is designed to be a lower layer for the semantic web stack that is 
extended by restriction.  All structure at this layer is imposed on all 
higher layers.  A layered structure is not necessary and the principle of 
minimal restriction suggests it should be omitted.

2) A further consideration is the cost of change at this point.  To switch 
to a layered approach would require a massive rethink and would affect not 
only the RDFCore specs but also OWL.  Only a show stopping problem with the 
current design could justify the cost of such a change.

We note that it is possible to build more strictly layered languages on top 
of RDF(S), Owl DL/Lite being examples.

Brian