“The first thing is deciding what the program should do. If you don’t think carefully about that, it’s going to wind up not doing something that it probably should do or you wind up with an ‘absolute bug-free program’. Because to have a bug, you have to have some notion of what it means for the program to be operating correctly. And there is no precise definition of what it means for the program to be correctly operating — so, well, no bugs. Not a very good situation.”