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
M. Huchard, A. Napoli, M. Hacene, and P. Valtchev. Selected Contributions in Data Analysis and Classification, Studies in Classification, Data Analysis, and Knowledge Organization, (October 2007)