Tests and Tests in Programming

When you have a well typed programme, one finds that testing does more to help you bring it into place. So there's a nice thing that happens, a kind of virtuous combination of the two. And then tests let you kind of test out what's going on at particular points. You could use logical tools like decision procedures to find how much of that have i covered? If i have not covered everything, give me an example state that was not covered, and then take that and see, can i cover it in my real execution? Can i get to it? What happens if i start running from that state?"