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Re: weird example in RDF-MT document...

From: Axel Polleres <axel.polleres@urjc.es>
Date: Fri, 26 Jan 2007 11:16:22 +0100
Message-ID: <45B9D4F6.1020503@urjc.es>
To: Alan Ruttenberg <alanruttenberg@gmail.com>
CC: www-rdf-logic@w3.org

Alan Ruttenberg wrote:
> On Jan 25, 2007, at 4:18 PM, Axel Polleres wrote:
>> (the blank node id in E isn't supposed to have any meaning in  common 
>> with _:yyy in the original graph).
> Maybe. Elsewhere in the document we have:
>> "This effectively treats all blank nodes as having the same meaning  
>> as existentially quantified variables in the RDF graph in which  they 
>> occur, and which have the scope of the entire graph. In terms  of the 
>> N-Triples syntax, this amounts to the convention that would  place the 
>> quantifiers just outside, or at the outer edge of, the N- Triples 
>> document corresponding to the graph. This in turn means  that there is 
>> a subtle but important distinction in meaning between  the operation 
>> of forming the union of two graphs and that of  forming the merge. The 
>> simple union of two graphs corresponds to  the conjunction ( 'and' ) 
>> of all the triples in the graphs,  maintaining the identity of any 
>> blank nodes which occur in both  graphs. This is appropriate when the 
>> information in the graphs  comes from a single source, or where one is 
>> derived from the other  by means of some valid inference process, as 
>> for example when  applying an inference rule to add a triple to a 
>> graph. Merging two  graphs treats the blank nodes in each graph as 
>> being existentially  quantified in that graph, so that no blank node 
>> from one graph is  allowed to stray into the scope of the other 
>> graph's surrounding  quantifier. This is appropriate when the graphs 
>> come from different  sources and there is no justification for 
>> assuming that a blank  node in one refers to the same entity as any 
>> blank node in the other."
> In other words, there seems to be a choice about how two graphs are  to 
> be considered together, in one case same name bnodes are allowed  to be 
> considered as referring to the same node, and in the other case  not. To 
> make your argument you would need to justify why the only  
> interpretation of the statements that is reasonable is one where the  
> bnodes are not considered to be the same node.


The spec is very clear here: Entailment of S and E has nothing to do 
with merge or union of S and E, it is about interpretations. The 
paragraph you cite only says that when you merge graphs you need to 
rename blanks whereas this is not the case for union.

Moreover, even if you add the triple
_:yyy <ex:property> <ex:foo> .
by union, this triple is redundant in the resultant non-lean graph.


> Immediately above the example you point out, it says:
>> "Since the relation between triples and reifications of triples in  
>> any RDF graph or graphs need not be one-to-one, asserting a  property 
>> about some entity described by a reification need not  entail that the 
>> same property holds of another such entity, even if  it has the same 
>> components."
> This seems to indicate that the choice is that the :_yyy in the  second 
> graph is the same node as the :_yyy in the first graph.
> That said, while I'm not sure whether it is an error, as you suggest,  
> the example could be constructed the way you later did, so as to  avoid 
> this potential confusion.
> Regards,
> Alan

Dr. Axel Polleres
email: axel@polleres.net  url: http://www.polleres.net/
Received on Friday, 26 January 2007 19:41:28 UTC

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