In my experience, drawing the state transition diagram is the core of the design. I remember having to upgrade an old bit of code that I had inherited, and the lack of documentation meant that in the end it was easier to reimplement that to modify. My project manager had been a bit perplexed at why the design was taking so long as, to quote "The slide goes in, the slide goes out". When I showed him the diagram with 12 states he understood why it took more than a day to code and test.

After drawing the state diagram it didn't really take very long to code up in C. So far I'm not making much of a case for CASE, but code maintenance is where CASE tools start to look good. If the original implementation had used a suitable tool, then the upgrade wouldn't have needed so much work. I'd like to think that the documentation I produced will be helpful in the future, but it's entirely possible that a future maintainer will modify the C and let the documentation lapse.