Two under-approximation techniques for 3-modal abstraction coverage of event systems: Joint effort?

Model-based testing of event systems can take advantage of considering abstractions rather than explicit models, for controlling their size. A test is then a connected and reachable event sequence. This paper reports on experiments made when adapting for event systems two known under-approximation techniques of predicate tri-modal (may, must+, must-) abstractions. We first instantiate all the abstract may transitions, preferably as reachable instances. Second, we complete this underapproximation with instantiations of Ball chains, i.e. sequences in the shape of must-∗may.must+∗ transitions, as such sequences are guaranteed to have connected instantiations. We present a backward symbolic instantiation algorithm for connecting these two under-approximations. We experimentally address the question of their complementarity. Surprisingly, our results show that Ball chains have not enhanced the coverage already achieved by the first step of may-transitions instantiation. However, it has enhanced the number of test steps by prolonging the already existing tests.