A new generation modechart verifier

We have implemented a new-generation modechart verifier called vf3. The new verifier is capable of dealing with modecharts with arbitrary hierarchy. More importantly, it employs several novel strategies to effectively reduce the search space while speeding up the verification process. It represents the computation graph for a modechart in a more compact form called a 'zone-based computation graph'. It uses on-the-fly deterministic pruning to reduce the outgoing edges from nodes in a graph. Finally, it builds on-the-fly a quotient graph based on a given real-time logic (RTL) formula. Our experimental results show that vf3 reduces the sizes of computation graphs by factors ranging from 2 to 10.