# RDF semantics: a question about instances and interpolation lemma

From: Chet Murthy <chet@watson.ibm.com>
Date: Mon, 02 Dec 2002 13:04:47 -0500
Message-Id: <200212021804.gB2I4lKT001593@nautilus-chet.watson.ibm.com>

```

[Not sure if this is the right place to send such a note.]

I was reading RDF-MT (http://www.w3.org/TR/2002/WD-rdf-mt-20021112/)
and it seems to me that there's a problem with the definition of
instances.  This might be trivial, and just an ambiguity in the spec.

Definition of instance:

An instance of an RDF graph is, intuitively, a similar graph in which
some blank nodes may have been replaced by urirefs or
literals. However, it is technically convenient to also allow blank
nodes to be replaced by other blank nodes, so we need to state this
rather more precisely. Say that one triple is an instance of another
if it can be obtained by substituting zero or more urirefs, literals
or blank nodes for blank nodes in the original; and that a graph is an
instance of another just when every triple in the first graph is an
instance of a triple in the second graph, and every triple in the
second graph has an instance in the first graph. Note that any graph
is an instance of itself.

By this definition,

(1a) eg:a eg:prop1 eg;b.
(1b) eg:c eg:prop2 eg:d.

is an instance of

(2a) _:a eg:prop1 _:b.
(2b) _:b eg:prop2 _:c.

since

(1a) is an instance of (2a) by { _:a -> eg:a, _:b -> eg:b }

(1b) is an instance of (2b) by { _:b -> eg:c, _:c -> eg:d }

Clearly, this shouldn't be allowed, but I can't find a reading of the
definition of instance which disallows it.  Equally, the model theory
seems to indicate (I haven't worked thru the details completely) that
this entailment should NOT hold.

I think I could formulate a modified definition of instance, to
restore the Interpolation Lemma, but, well, I figured I'd check and
see if I was barking up the wrong tree ...

Thanks,
--chet--
```
Received on Monday, 2 December 2002 21:59:29 UTC

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