Use and abuse of occurrence RE: [topicmapmail] Are Facets Really Simple After All?

[Jan Algermissen]

Bernard Vatant wrote:

> Rather than trying
> to interpret whatever semantics are or are not to be found in the
> definition of occurrence in various versions of the standard, let's face
> some pragmatic evidence. Everything relevant to the topic has to be
> attached to it in some way. Beside names and subject identifiers, TM
> provides only two ways to do that. Either you use an association, or you
> use an occurrence. In any case where you can't or don't want to use an
> association, using occurrence is the default way. So the spectrum of
> occurrence use has spread from the ones fitting with the original BOTB
> index intention, to more and more exotic, and you say *heretic* ones.

Bernard--

just because <occurrence> is the only way to attach a property to a topic
in XTM doesn't mean that it is correct to use it as such. Let's distinguish
between:

a) XTM is broken because it does not provide a facility to attach mere
   properties to as topic

b) <occurrence> really has the semantics that make it suitable for
   using it to assign properties to a topic.

If it is b) then we are fine but if it is a) I'd rather have XTM extended
(fixed) than to carry this 'error' onward.

I am deeply convinced that a) is correct and that XTM should be extended
with an element that allows assigning property value pairs to topics.

What do others (especially authors of topic maps, not XTM software developers)
think about this?


Some issues to consider:

* All interpretations of the <occurrence> element I have seen so far
  (PMTM4, various APIs, the current SAM draft) regard an occurrence
  (that is: the relationship between topic and the other end) as someting
  with identity (occurrences have type(s), scope, are represented as
  objects in their own right, etc). This conceptual and implementation
  overhead does not seem to match the semantics of a simple property
  (e.g. "Jim weighs 150kg")

* Two very important properties of topics: the SubjectAdress and the
  SubjectIndicators are *not* interpreted with the complexity of
  an occurrence - they are understood to be simply propety class/value
  pairs attached to a topic. So, why the overhead for "Jim's age is 34"
  but not for "Jim has {URI} as the set of subject indicators". Makes
  not much sense to me.

* XTM is just a single format for representing topic map information,
  I don't think it is wise to use XTM as a base for arguing. IOW: if
  we allow XTM to dictate that any abstract model for topic maps does
  not have a property facility we impose unneccessary complexity on
  interpretations of other formats. Consider processing RDF represented
  Dublin Core into a topic map....is it really clever to insist that all
  attributes of a resource are to be represented as occurrences? Just
  because XTM lacks a property facility?
 
* Introducing a new element does not harm existing XTM documents, they
  can easily be transformed into instances of the new DTD.


Jan

-- 
Jan Algermissen                           http://www.topicmapping.com
Consultant & Programmer	                  http://www.gooseworks.org