Enhancing top-down solving with widening and narrowing

Abstract

We present an enhancement of the generic fixpoint algorithm TD which can deal with widening and narrowing even for non-monotonic systems of equations. In contrast to corresponding enhancements proposed for other standard fixpoint algorithms, no extra priorities on variables are required. Still, a mechanism can be devised so that occurrences of the widening/narrowing operator are inserted as well as removed dynamically.