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Re: Layers

From: David Wood <david@3roundstones.com>
Date: Wed, 2 May 2012 08:30:44 -0400
Cc: Thomas Baker <tom@tombaker.org>, Dan Brickley <danbri@danbri.org>, Sandro Hawke <sandro@w3.org>, public-rdf-wg <public-rdf-wg@w3.org>
Message-Id: <231A8DAD-AE58-47F8-BBD8-5AA235AE311E@3roundstones.com>
To: Pat Hayes <phayes@ihmc.us>
Hi Pat,

On May 2, 2012, at 24:12, Pat Hayes wrote:
> On Apr 30, 2012, at 5:00 PM, Thomas Baker wrote:
>> On Mon, Apr 30, 2012 at 10:01:23PM +0200, Dan Brickley wrote:
>>>> A surface is something like a bundle of overlaid layers, like a movie still
>>>> made up from overlaid cels. Or, put the other way, a layer is part of a
>>>> suface which has been peeled off and raised above the surface. (Danbri's
>>>> pictures work either way.) Think of the surface as the opaque background of
>>>> the transparent layers: bnodes are marks on this surface. We could copy a
>>>> layer onto another surface, for example, and then the suface idea of
>>>> conjunction (replacing unions and merges) works here also: to conjoin A and
>>>> B, just copy them both onto a new surface.
>>> That's quite interesting. It's refreshing to be able to start to articulate
>>> operations that work with larger units than triples. I wonder what other
>>> kinds of operations between graphs/layers/surfaces are useful in everyday rdf
>>> (/owl) life?
>> The Layers metaphor fits very nicely with the notion of Levels in the FRBR use
>> case as described at [1]:
>>   This proposal views descriptions of WEMI entities as bundles of statements
>>   made at different levels of abstraction, from the most concrete Item level
>>   to the most abstract Work level.
> Hmmmm. Sorry to be a bit of a pooper, but this worries me. When Sandro + Danbri came up with the "levels" terminology, I hated it because it suggested that a "vertical" dimension between layers was meaningful and even important, which it isn't. It has no semantic meaning at all: the 'layers' have no intrinsic ordering, and in fact are best thought of as constituting a set rather than a sequence or list. But Sandro calmed me down by saying, no, its just that they are transparent, like sheets of celluloid. But my worry has now reared its head again, because WEMI "levels" really do have a vertical order, and one that is semantically significant (level of abstraction).  If we endorse this kind of usage, I am afraid that others will start using the "depth" of a "layer" to mean time sequence, where deeper means older; and others to mean importance, where deeper means less trusted; and others to mean all kinds of other things. Which just re-creates the kind of confused ad-hocery that we have now. 
> Its OK to use layers to handle levels, supported by a suitable ontology maybe, but what I DONT want us to even hint at doing is to encourage people to use some kind of ordering of layers based on an implementation accident or something meaningless like lexical ordering of the "graph names" to encode anything meaningful. 

Even in the graphics editors analogy, layers may be reordered at will and have no intrinsic meaningful order.  Ordering does not appear to be part of the proposal in any way.

However, it seems that we have at least one use case (FRBR) that could be settled by ordering.  I'm OK with that as long as, as you say, any ordering happens above the RDF level (either via an ontology or another form of social contract).


> Pat
>> Multi-level WEMI descriptions specify the
>>   characteristics that any given Item shares with other Items at the level of
>>   Work, Expression, and Manifestation.  Ideally, it would be possible to
>>   incorporate descriptions of resources at the Work, Expression, and
>>   Manifestation levels, maintained in a distributed manner by various
>>   institutions, into the local descriptions of particular Items.  
>> (In our discussion a few weeks ago, David wondered whether the inferencing
>> requirement expressed therein suggests the need for a graph that holds named
>> graphs (nesting) [2].  I'm wondering whether the Layers idea re-frames that 
>> question.)
>> Tom
>> [1] http://lists.w3.org/Archives/Public/public-rdf-comments/2012Apr/0001.html
>> [2] http://www.w3.org/2012/04/11-rdf-wg-irc#T16-15-09
>> -- 
>> Tom Baker <tom@tombaker.org>
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Received on Wednesday, 2 May 2012 12:31:22 UTC

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