Ontology on PostgreSQL - is there something?

Dear PG-hackers,

Based on the paper below [1], I ask: is there anyone working on, or already
tried to such native implementation on PostgreSQL? I didn't find anything
related on pgFoundry. There is also a presentation [2] related to the paper.

"By Souripriya Das, Eugene Inseok Chong, George Eadon, Jagannathan
Srinivasan, Proceedings of the 30th VLDB Conference, Toronto, Canada.
Ontologies are increasingly being used to build applications that utilize
domain-specific knowledge. This paper addresses the problem of supporting
ontology-based semantic matching in RDBMS. The approach enables users to
reference ontology data directly from SQL using the semantic match
operators, thereby opening up possibilities of combining with other
operations such as joins as well as making the ontology-driven applications
easy to develop and efficient. In contrast, other approaches use RDBMS only
for storage of ontologies and querying of ontology data is typically done
via APIs. This paper presents the ontology-related functionality including
inferencing, discusses how it is implemented on top of Oracle RDBMS, and
illustrates the usage with several database applications."

[1]
http://www.cs.toronto.edu/vldb04/protected/eProceedings/contents/pdf/IND3P1.PDF
[2] http://www-scf.usc.edu/~csci586/ppt-2005/dguo.pps

Besides, what are your opinions on the subject?

Is it worthful to have such native implementation on PG, even as a modest
contrib module?
Based on your development know-how on the backend internals, how difficult
is it to develop this (3 operators + 1 index type)?

--
Best regards,

Rodrigo Hjort
http://icewall.org/~hjort
CELEPAR - Cia de Informática do Paraná - Brasil
http://www.pr.gov.br