  This paper offers an approach to extensible knowledge representation and
reasoning for a family of formalisms known as Description Logics. The approach
is based on the notion of adding new concept constructors, and includes a
heuristic methodology for specifying the desired extensions, as well as a
modularized software architecture that supports implementing extensions. The
architecture detailed here falls in the normalize-compared paradigm, and
supports both intentional reasoning (subsumption) involving concepts, and
extensional reasoning involving individuals after incremental updates to the
knowledge base. The resulting approach can be used to extend the reasoner with
specialized notions that are motivated by specific problems or application
areas, such as reasoning about dates, plans, etc. In addition, it provides an
opportunity to implement constructors that are not currently yet sufficiently
well understood theoretically, but are needed in practice. Also, for
constructors that are provably hard to reason with (e.g., ones whose presence
would lead to undecidability), it allows the implementation of incomplete
reasoners where the incompleteness is tailored to be acceptable for the
application at hand.
