We extend the description logic EL++ with reflexive roles
and range restrictions, and show that subsumption remains tractable if
a certain syntactic restriction is adopted. We also show that subsumption
becomes PSpace-hard (resp. undecidable) if this restriction is weakened
(resp. dropped). Additionally, we prove that tractability is lost when
symmetric roles are added: in this case, subsumption becomes ExpTime-
hard
I. Niles, and A. Pease. Proceedings of the International Conference on Formal Ontology in Information Systems - Volume 2001, page 2--9. New York, NY, USA, ACM, (2001)