[topicmapmail] Interpretive Difference between Occurrences and SI or SL

[Lars Heuer]

Hi Tobias,

> What is the interpretive difference between occurrences and subject
> indicators or locators. Both uses URIs to indicate relevant resources.

Very simple: An occurrence has no meaning for the identity of a topic.

An example:
   Given that you've a webpage "My dead pets" which lists all your
   pets which are (for some or the other reason) are dead:
   <http://meshed.de/my-dead-pets>
   
   Additionally you have setup a set of PSIs for each pet:
   - <http://psi.meshed.de/pets/bello>
   - <http://psi.meshed.de/pets/mietzi>
   ...

   Now you can refer to "Bello" with the subject identifier
   "http://psi.meshed.de/pets/bello" and to "Mietzi" with the subject
   identifier "http://psi.meshed.de/pets/mietzi".

   Both, Bello and Mietzi have the same occurrence of type "website"
   (or whatever) <http://meshed.de/my-dead-pets>

   But Bello and Mietzi wouldn't be the same subject.

   If you ask your favourite Topic Maps engine: Give me every topic
   with an occurrence value <http://meshed.de/my-dead-pets>, Mietzi
   and Bello will be the result.


[...]
> The subject indicator describes the subject, but there are a lot of
> cases where the indicator is the one and only occurrence of the topic.

You can, if you want, add the subject identifier IRI to the
occurrences. I.e. if you use Wikipedia as source for PSIs, you can say

   %prefix wp http://en.wikipedia.org/wiki/

   wp:John_Lennon
   description: wp:John_Lennon

   -> Result: A topic with the subject identifier
   <http://en.wikipedia.org/wiki/John_Lennon> and an occurrence of
   type "description" with the value
   <http://en.wikipedia.org/wiki/John_Lennon>


> This also leads to the question: What to do with topics where the si
> equals the occurrence and how to model them? What means this?

No meaning, TMDM-wise, identity-wise.

Best regards,
Lars
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