Why only object properties?
On the question of whether sub-property chains are only made up of object properties (and whether a punned datatype property's values might be carried along). From Uli Sattler's email.
There are reasons why sub-property chains are only made up of object properties: decidability in OWL (DL and 1.1) relies on the fact that "datatype consistency" can be checked for each object separately, without referring to other objects and the values of their datatype properties. If we would need to do this, we would more likely be in trouble, and would need to
- be much more careful about what datatypes and datatype predicates to allow without loosing decidability and
- use more complex reasoning mechanisms that have, to the best of my knowledge, only been described on paper and never been implemented or tested.
Note, however, that it may be possible to have a limited form of datatype properties on chains. Issue 8 notes that Uli Sattler will think about whether a non structural restriction preventing use of non-unary predicates other than = makes the use of them decidable.
If you want to know more, check out
- A Tableau Algorithm for Description Logics with Concrete Domains and General TBoxes. Carsten Lutz and Maja Milicic. Journal of Automated Reasoning. To appear.
- Description Logics with Concrete Domains - A Survey, Carsten Lutz. In Philippe Balbiani, Nobu-Yuki Suzuki, Frank Wolter, and Michael Zakharyaschev, editors, Advances in Modal Logics Volume 4. King's College Publications, 2003.