I gratefully acknowledge help received from Nancy
Leveson, Willem-Paul de Roever, Al Mok, Dave Parnas, Murray Wonham, Jozef
Hooman, Ron Koymans, Ben Moszkowski, Mike Gordon, Farnam Jahanian, Insup Lee,
Rajeev Alur and Roger Hale. Of course, any errors or misinterpretation
of concepts is my sole responsibility.