W3C home > Mailing lists > Public > www-rdf-logic@w3.org > January 2007

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

From: Alan Ruttenberg <alanruttenberg@gmail.com>
Date: Fri, 26 Jan 2007 01:24:34 -0500
Message-Id: <90E1598F-0607-4B17-B32F-12A1FD5B270E@gmail.com>
Cc: www-rdf-logic@w3.org, Denny Vrandecic <dvr@aifb.uni-karlsruhe.de>
To: axel@polleres.net, Axel Polleres <axel.polleres@urjc.es>

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.

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.

Received on Friday, 26 January 2007 06:36:00 UTC

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