nit: On my first read, I was a bit confused trying to solve the puzzle (without TLA+) because the second question was underspecified:
> If the algorithm terminates with b = 1, what must have been true at the start?
For anyone that wasn't clear, "b" isn't the number of beans in general, it's specifically the the number of black beans. Introducing a variable before defining what it is can be a bit confusing, hope this comment helps others.
> If the algorithm terminates with b = 1, what must have been true at the start?
For anyone that wasn't clear, "b" isn't the number of beans in general, it's specifically the the number of black beans. Introducing a variable before defining what it is can be a bit confusing, hope this comment helps others.