You can't prove anything about a program written in C or FØRTRAN. It's really just Peek and Poke with some syntactic sugar.
That lack of programmability is probably what ultimately will doom vi. It can't extend its domain.
I think multiple levels of undo would be wonderful, too.
Sometimes the easiest way to get something done is to be a little naive about it.
Take responsibility for the things you build and invent.
The standard definition of AI is that which we don't understand.