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Re: horst-01 - after hours discussion

From: Jeremy Carroll <jjc@hplb.hpl.hp.com>
Date: Mon, 23 Jun 2003 14:47:51 +0100
Message-ID: <3EF70507.5080900@hplb.hpl.hp.com>
To: Jeremy Carroll <jjc@hplb.hpl.hp.com>
CC: w3c-rdfcore-wg@w3.org

Jeremy Carroll wrote

> I am not expressing a personal opinion on the content here.

Having discussed this minute with Dave Reynolds, who leads on the Jena RDFS 
implementation, I now express what will pass for the HP opinion, which 
comes to a mild preference for the treatment in the current editors draft 
over the changes discussed in this minute.

> The current design has extensional definitions of subClassOf and
> subPropertyOf.
> http://www.ihmc.us/users/phayes/RDF_Semant_Edit_appenda.html#rdfs_interp
> [[
> <x,y> is in IEXT(I(rdfs:subClassOf)) if and only if x and y are in IC and
> ICEXT(x) is a subset of ICEXT(y)
> <x,y> is in IEXT(I(rdfs:subPropertyOf)) if and only if x and y are in IP and
> IEXT(x) is a subset of IEXT(y)
> ]]
> This could be changed to intensional definitions
> [[
> If <x,y> is in IEXT(I(rdfs:subClassOf)) then x and y are in IC and ICEXT(x)
> is a subset of ICEXT(y)
> If <x,y> is in IEXT(I(rdfs:subPropertyOf))then x and y are in IP and IEXT(x)
> is a subset of IEXT(y)
> ]]
> transitivity and reflexivity can be achieved by fiat:
> e.g.
> [[
> IEXT(I(rdfs:subClassOf)) and IEXT(I(rdfs:subPropertyOf)) are
> transitive and reflexive.
> ]]
> (in the green box)

It seems like rather a big change to make at this late stage. Would you be 
confident there weren't other properties that need to be declared by fiat 
that haven't yet been identified?

> The rationale for such a change comes in the entailment rule appendix.
> The bizarre rules rdfs12a and rdfs12b would no longer be needed, since their
> validity depends on the extensional reading of subClassOf.

A further advantage is that it may make it less likely that someone will 
find an additional corner case like ter Horst's.

> They would hence be moved from the main RDFS section to the variant at the
> end (with rdfs2a rdfs3a rdfs4a' and rdfs4b'). Moreover, the completeness fix
> I noted in my review would then be added to the only occurrence of these
> rules.
> Since these rules are a significant obstacle to complete implementation of
> RDFS, dropping them to the optional extras section, is an advantage.

This rules may be a nuisance but they are not a major obstacle.
They don't look difficult or expensive to implement.

Given that the result of the rewrite would be, in some sense,
less clean I would think it would need some additional motivation than just
moving a couple of obscure rules slightly further down in the appendix.

Jeremy (with help from Dave Reynolds)
Received on Monday, 23 June 2003 09:48:21 UTC

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