A key technique for the verification of programs is counterexample-guided abstraction refinement (CEGAR). In a previous approach,
we developed a CEGAR-based algorithm for the modal μ-calculus, where refinement applies only locally, i.e. lazy abstraction techniques are used. Unfortunately, our previous algorithm was not completely lazy and had some further drawbacks, like a
possible local state explosion. In this paper, we present an improved algorithm that maintains all advantages of our previous
algorithm but eliminates all its drawbacks. The improvements were only possible by changing the philosophy of refinement from
state splitting into the new philosophy of state focusing, where the states that are about to be split are not removed.
This work is financially supported by the DFG projects (FE 942/2-1) and (SFB/TR 14 AVACS).