Besides, what's wrong with saying, without using another URI:

<http://en.wikipedia.org/wiki/Resource_Description_Framework> foaf:topic _:rdf .
<http://www.w3.org/RDF/> foaf:topic _:rdf .

This is an option I've been pushing here and there for months, without much feedback so far. I think this is the best one can do in RDF to capture the fact that two resources deal with the same thing/topic/subject, whereas keeping agnostic about what this thing actually "is".
The argument against it, that _:rdf is only a local node and therefore limits the usability of the declaration to the local file is moot IMO.

Suppose I have in another RDF file

<http://xmlfr.org/documentations/tutoriels/041015-0001> foaf:topic  _:b
<http://www.w3.org/RDF/> foaf:topic _:b .

Agreed in this case I cannot infer that _:b and _:rdf are the same, because foaf:topic bears no cardinality constraint.
But one can use some functional property instead, like skos:primarySubject, so if I have somewhere :

<http://en.wikipedia.org/wiki/Resource_Description_Framework> skos:primarySubject     _:b1
<http://www.w3.org/RDF/> skos:primarySubject     _:b1

And in another RDF graph

<http://xmlfr.org/documentations/tutoriels/041015-0001> skos:primarySubject     _:b2
<http://www.w3.org/RDF/> skos:primarySubject     _:b2

An agent can reasonably infer that _:b1 and _:b2 are the same and merge the three resources in a third file

<http://en.wikipedia.org/wiki/Resource_Description_Framework> skos:primarySubject     _:b
<http://www.w3.org/RDF/> skos:primarySubject     _:b
<http://xmlfr.org/documentations/tutoriels/041015-0001> skos:primarySubject     _:b

More about it at [1]

[1] http://universimmedia.blogspot.com/2006/04/more-use-cases-for-nondescript.html

Bernard Vatant

Bernard Vatant

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