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Re: Proposal for ISSUE-40 Skolemization

From: Pat Hayes <phayes@ihmc.us>
Date: Thu, 19 May 2011 23:48:54 -0500
Cc: <alexhall@revelytix.com>, <richard@cyganiak.de>, <public-rdf-wg@w3.org>
Message-Id: <45012DDE-2143-4F88-A274-C0FE9DB8CEC8@ihmc.us>
To: Peter Frederick Patel-Schneider <pfps@research.bell-labs.com>

On May 19, 2011, at 11:24 AM, Peter Frederick Patel-Schneider wrote:

> From: Alex Hall <alexhall@revelytix.com>
> Subject: Re: Proposal for ISSUE-40 Skolemization
> Date: Thu, 19 May 2011 09:16:03 -0500
> 
>> On Wed, May 18, 2011 at 7:02 PM, Peter Frederick Patel-Schneider
>> <pfps@research.bell-labs.com> wrote:
>> 
>>> 
>> From: Richard Cyganiak <richard@cyganiak.de>
>>> Subject: Re: Proposal for ISSUE-40 Skolemization
>>> 
>> Date: Wed, 18 May 2011 17:12:32 -0500
>>> 
>> 
>>>> On 18 May 2011, at 20:20, Peter Frederick Patel-Schneider wrote:
>>>>>>> [[ADD: Implementors should realize that this transformation
>> changes the meaning
>>>>>>> of an RDF graph (but this change is generally not harmful).]]
>>>>>> 
>>>>>> That sounds a bit scary. Perhaps:
>>>>>> 
>>>>>> [[ADD: This transformation does not change the meaning of an RDF
>>>>>> graph, except “using up” the Skolem IRI.]]
>>>>> 
>>>>> But this isn't true.
>>>> 
>>>> Grumble.
>>>> 
>>>> How about this?
>>>> 
>>>> [[ADD: This transformation slightly changes the meaning of an RDF
>>>> graph, because it “fixes” what the Skolem IRI identifies. See the
>>>> Skolemization Lemma in [RDF-Semantics] for a detailed technical
>>>> discussion.]]
>>> 
>>> 
>> I'm not keen on this, either.  I think that it needs further fixing.
>>> :-)
>> 
>> Would it be correct to say that the graph that results from replacing
>> blank nodes with skolem IRIs simple-entails the original graph? 
> 
> Yes, this is true.

And the skolemized graph is consistent with any other graph (not containing the skolem URis) exactly when the original graph is. Which is about as close as one can get to mutual entailment without actually having entailment.

Pat

> 
>> If so,
>> is
>> that a useful thing to say here?
> 
> Well, you probably want to say something like it is also very close in
> meaning to the original graph, but one has to be careful how to state
> this.
> 
> 
>> -Alex
> 
> peter

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Received on Friday, 20 May 2011 04:49:30 GMT

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