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
S. Grimm, B. Motik, and C. Preist. Proc. of the 3rd European Semantic Web Conference (ESWC 2006), volume 4011 of LNCS, page 575--589. Budva, Montenegro, Springer, (June 2006)