Minutes: XDI TC Telecon Thursday 1-2PM PT 2009-03-05

["Drummond Reed" <drummond.reed@cordance.net>]

Following are the minutes of the unofficial telecon of the XDI TC at:

Date:  Thursday, 05 March 2009 USA
Time:  1:00PM - 2:00PM Pacific Time (21:00-22:00 UTC)

ATTENDING

Bill Barnhill
Markus Sabadello
Mike Mell
Drummond Reed
Giovanni Bartolomeo
Nick Nicholas 
John Bradley


1) CONTINUE +X/+Y/+X+Y DISCUSSION

See Giovanni's original post at:

	http://lists.oasis-open.org/archives/xdi/200902/msg00034.html

And Bill's replies at:

	http://lists.oasis-open.org/archives/xdi/200902/msg00037.html
	http://lists.oasis-open.org/archives/xdi/200902/msg00038.html 


We started with the first link above and the example:

	*bugs.bunny/$has/+animal

Giovanni did not understand the semantics behind this. Bill said that $has
is very close to the SKOS "narrows" predicate (skos:narrows).

Drummond explained that =bill+dog represents the metagraph statement
=bill/$has/+dog. =bill+dog*fido would refer to an instance of =bill+dog. In
the first case, =bill+dog narrows the set of all dogs to those Bill has. In
the second case, =bill+dog*fido narrows the set of Bill's dogs to the
instance Fido.

John pointed out that so far we have not said that XRIs representing XDI
statements (e.g., $has statements) are authoritative. For example, we could
say =big+dog/$is$a/+fish. However Drummond spoke in favor of treating all
XRIs in the graph as statements, i.e., as part of the graph.

Giovanni was worried that *bugs.bunny+animal is not narrowing *bugs.bunny,
but broadening it. Drummond clarified that *bug.bunny is narrowing +animal,
not the other way around. He compared it *bugs.bunny+animal to
+animal*bugs.bunny. The first one is a specialization of animal, the second
one is an instance of an animal.

Bill suggested that we start a wiki page on XDI description logic. Drummond
noted that the last time this came up, the question was: is XDI a
description logic, or is it a syntax that could be used to express any
description logic?

Bill explained that any description logic (DL) that is of equal or greater
expressive power as another DL can be used to express that other DL. He said
we would need to define our operators, such as $and, $or, etc. Drummond
agreed and welcomed that, but said he doesn't want it to slow down the
fundamental specs. Giovanni agreed that it is very important to document any
DL that is inherent in the XDI RDF metagraph vocabulary, as well as any DL
that can be expressed using it.

We moved on to the next point in Giovanni's original email, which quoted
Drummond's email to say:

*** BEGIN QUOTE ***

And the inverse is true too. Classes can be used to restrict instances.

+car/$has/*monte.carlo          -->     +car*monte.carlo
+city/$has/*monte.carlo         -->     +city*monte.carlo

*** END QUOTE ***

In the first case Monte Carlo is an instance of a car, and in the second it
is an instance of a city. Giovanni had replied saying he sees this as an
interesting possibility. We can use $has to enumerate members of a class.
And we can use $has to enumerate *properties* of instances =drummond+friend,
=drummond+home, which are "classes" +friend +home etc., using then the
pattern class-instance to identify which one is the right instance.

Giovanni gave the examples of:

	=drummond+home*san.diego
	=drummond+home*los.angeles 
	=drummond+home!1234.5678.9012
	=drummond+home!9876.5432.2109

Giovanni quoted from Drummond's mail:

*** BEGIN QUOTE ***

From all of these, my conclusion is that it is not a problem for +x/$has/+y
to infer both +x+y/$is$a/+y and +x/+y/+x+y. The first expresses inheritance
(subclassing)...

*** END QUOTE ***

Giovanni's reply said that is probably true when +y is a class. Some other
times it might not: what does +city*monte.carlo/$is$a/*monte.carlo mean?
Since *monte.carlo is an instance, in this statement it is not in the right 
place, assuming that the meaning of $is$a is "subclass of" or "instance of".

Drummond said the way he interprets this is that
+city*monte.carlo/$is$a/*monte.carlo means that, because *monte.carlo is an
instance, this statement is saying it is an instance (i.e., the object is
*monte.carlo).

Bill pointed to his email to the list on this topic - that instances can be
considered classes in themselves. Giovanni referred to this as a
"metalevel".

ACTION ITEMS

# ALL - review Giovanni's email.
# BILL AND GIOVANNI - put these emails into a wiki page format for
documenting a DL.
# BILL AND GIOVANNI - arrange a chat between the two of them to move that
forward.