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Re: ISSUE-180: Last Call Comment: PFWG: skosxl:Label class

From: Alistair Miles <alistair.miles@zoo.ox.ac.uk>
Date: Thu, 23 Oct 2008 11:36:44 +0100
To: public-swd-wg@w3.org
Cc: public-esw-thes@w3.org
Message-ID: <20081023103644.GI9010@skiathos>

The second part of this comment needs some discussion before I can
frame a response...

Executive summary:

It would be a good thing if the SKOS XL vocabulary could serve as an
extension point for the provision of labels in other modalities, for
example in accessibility-related use cases.

To serve as an extension point, we have to either

(1) live with the current XL data model, especially the restriction on
the cardinality of skosxl:literalForm, which requires that all
instances of skosxl:Label have exactly one plain literal form (even if
they are intended to convey a label in another modality).

or

(2) relax the restriction on skosxl:literalForm, such that instances
of skosxl:Label are not required to have a plain literal form (but if
they do, they have at most 1)

Personally, I think I favour (2), although it requires a substantive
change to the SKOS Reference.

Further discussion below...

On Wed, Oct 08, 2008 at 09:54:57AM +0000, SWD Issue Tracker wrote:
> 
> 
> ISSUE-180: Last Call Comment: PFWG: skosxl:Label class
> 
> http://www.w3.org/2006/07/SWD/track/issues/180
> 
> Raised by:  Everyone
> On product: All
> 
> Raised by Al Gilman on behalf of PFWG in [1]:
> 
> """
[...]
> The label framework should explicitly cater for non-textual labels in  
> image, audio or video format, and as provided in other markup  
> languages such as MathML.  Labels in other modalities may serve as  
> alternate labels in accessibility-related use cases.  SKOS should  
> provide guidance as to how to provide images, audio and video content  
> as alternate labels.  Currently, icons are being standardized as  
> representing concepts in an upcoming multi-part standard ISO/IEC  
> 11581, developed by ISO/IEC JTC1 SC35.  SKOS should be able to  
> specify these icons as part of a knowledge organization system.
[...]
> """

Currently the SKOS Reference defines skosxl:Label as "a special class
of lexical entities". It also says that "each instance of this class
has a single plain literal form...". 

This is reinforced by statement S53, where skosxl:Label is a sub-class
of a restriction on skosxl:literalForm cardinality exactly 1.

This raises a possible problem for use of the XL vocabulary as an
extension point for extensions as described by Al above.

For example, let's say a third party wants to extend the SKOS XL
vocabulary to represent labels in various XML markup languages. They
define extensions as follows:

ex:XMLLabel rdf:type owl:Class ; 
  rdfs:subClassOf skosxl:Label .

ex:xmlLiteralForm rdf:type owl:DatatypeProperty ; 
  rdfs:domain ex:XMLLabel ; 
  rdfs:range rdfs:XMLLiteral .

ex:xmlContentType rdf:type owl:DatatypeProperty ;
  rdfs:domain ex:XMLLabel .

to be used as in e.g.

<MyConcept> skosxl:prefLabel <MyXMLLabel> .

<MyXMLLabel> rdf:type ex:XMLLabel ;
  ex:xmlLiteralForm """E = mc<sup>2</sup>""" ;
  ex:xmlContentType "application/xhtml+xml" .

The potential problem is that the XL data model currently requires
that *every* instance of xl:Label has a plain literal form. Therefore,
the example above entails

<MyXMLLabel> skosxl:literalForm _:aaa .

where _:aaa is some as yet unknown plain literal.

In practical terms, this means that every refinement of the
skosxl:Label class should always give a plain literal form for the
label, in addition to whatever other modality is the primary carrier
of the label.

>From an accessibility point of view this is not necessarily a bad
thing. I.e. it's very roughly analogous to requiring alt text for
images in HTML. However it is a bit restrictive for all cases.

The alternative would be to relax the definition of the xl:Label
class. So instead of

""" A special class of lexical entities, called skosxl:Label, is
defined. Each instance of this class has a single RDF plain literal
form, but two instances of this class are not necessarily the same
individual if they share the same literal form. """

we say something like

""" This appendix defines a class called skosxl:Label. Each instance
of this class has at most one RDF plain literal form, but two
instances of this class are not necessarily the same individual if
they share the same literal form. """

and we replace S53 with either

"""
skosxl:Label is a sub-class of a restriction on skosxl:literalForm cardinality at most 1.
"""

or (reverting to the previous model)

"""
skosxl:literalForm is an instance of owl:FunctionalProperty.
"""

What do you think?

Cheers,

Alistair.

[1] http://lists.w3.org/Archives/Public/public-swd-wg/2008Oct/0063.html
 
-- 
Alistair Miles
Senior Computing Officer
Image Bioinformatics Research Group
Department of Zoology
The Tinbergen Building
University of Oxford
South Parks Road
Oxford
OX1 3PS
United Kingdom
Web: http://purl.org/net/aliman
Email: alistair.miles@zoo.ox.ac.uk
Tel: +44 (0)1865 281993
Received on Thursday, 23 October 2008 10:37:32 GMT

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