  Software Model Checkers have shown outstanding performance improvements in
recent times. Moreover, for specific use cases, formal verification techniques
have shown to be highly effective, leading to a number of high-profile success
stories. However, widespread adoption remains unlikely in the short term and
one of the remaining obstacles in that direction is the vast number of
instances which software model checkers cannot fully analyze within reasonable
memory and CPU bounds. The majority of verification tools fail to provide a
measure of progress or any intermediate verification result when such
situations occur.
  Inspired in the success that coverage metrics have achieved in industry, we
propose to adapt the definition of coverage to the context of verification. We
discuss some of the challenges in pinning down a definition that resembles the
deeply rooted semantics of test coverage. Subsequently we propose a definition
for a broad family of verification techniques: those based on Abstract
Reachability Trees. Moreover, we discuss a general approach to computing an
under-approximation of such metric and a specific heuristic to improve the
performance.
  Finally, we conduct an empirical evaluation to assess the viability of our
approach.

Software Engineering