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Abstract:
An extension of the concept description language ${\cal alc\}$
used in {\sc kl-one}-like terminological reasoning is presented.
The extension includes multi--modal operators that can either
stand for the usual role quantifications or for modalities such as
belief, time etc.
The modal operators can be used at all
levels of the concept terms, and they can be used to modify
both concepts and roles. This is an instance of a new kind of
combination of modal logics where the modal
operators of one logic may operate
directly on the operators of the other logic.
Different versions of this logic are investigated and various results
about decidability and undecidability are presented. The main
problem, however, decidability of the basic version of the logic,
remains open.
