From: Sandro Hawke (sandro@w3.org)
Date: 10/24/01
> Richard Fikes wrote:
>
> > >The knowledge base contains the statements: "Pat's car is blue, and
> > >there is something colored red." Somewhat more formally:
> > >
> > > RDF(PatsCar, color, blue).
> > > exists x (RDF(x, color, red)).
> >
> > Sorry to be dense, but how does one state "there is something colored
> > red" in DAML+OIL?
Deborah McGuinness wrote:
> there are a few ways, arguably, none perfect.
>
> the most direct is mincardinalityQ
> state that something has mincardinality 1 (or more)
> and the Q in this case is RED-THING
>...
> Deborah L. McGuinness
There is a much simpler way, since RDF includes existential variables.
<rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
xmlns:example="http://example.com#">
<rdf:Description>
<example:color rdf:resource="http://example.com#red"/>
</rdf:Description>
</rdf:RDF>
This is parsed by the validator [1] to
_:j70241 <http://example.com#color> <http://example.com#red> .
As Pat's model theory [2] (and his various e-mails to me on
www-rdf-rules) make clear, the "unlabeled" nodes (like _:j70241)
should be interpretted as existential variables.
This clarification to RDF is fairly recent, and not yet official,
(it's just a working draft), but I haven't heard anyone objecting yet.
I have heard several members of the original RDF group say this is
what they meant.
-- sandro
[1] http://www.w3.org/RDF/Validator/
[2] http://www.w3.org/TR/rdf-mt/
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