The Problem With a Positive Negative Counter

The idea is that the three and the five, if the number can move in both directions, don't allow us to know whether you've seen events and moved past them or haven't seen them at all. One of the invariants that's required to make that true is that actors can only modify their position in the set. So our five, if we both can concurrently modify the same entry in the vector, that five is not safe by itself. We need more information.