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
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D. Frankel, P. Hayes, E. Kendall, and D. McGuinness. 1st International Workshop on the Model-Driven Semantic Web (MDSW2004), Monterey, California, USA, (September 2004)