Attempts to combine some sort of rules with a description logic go back at least as far as the Classic system [2] . Starting in the late 1990s, there were a number of attempts to combine Datalog (as the premier deductive database language) with description logics; notable examples include AL-log [6] and CARIN [22] . While partially motivated by the desire to increase the expressive power of both components, these attempts (esp. AL-log) at hybrid systems were strongly constrained by the desire

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... retain both the modeling and the computational properties of the respective components, and even, perhaps, the implementation techniques or actual implementations. (For example, the proof procedure for AL-log described in [6] calls an independently developed description logic reasoner as a oracle.) In general, when considering combination formalisms, the upper bound is the unrestricted union of the two systems. In essence, the Semantic Web Rule Language (SWRL) 3 [13] is the unrestricted union of OWL DL (i.e., roughly the description logic SHOIN (D)) and (binary) function-free Horn logic. The result is a very expressive formalism which is, unsurprisingly, undecidable. Also, it is comparatively not well understood: there are no native reasoners for it, relationships to other formalisms are not precisely mapped out, and there is almost no experience in using SWRL for ontology modeling. However, it does serve as a unifying overarching formalism for various rule and rule like extension to OWL DL. It is, for example, a superset of (binary) AL-log and of CARIN.