ABA counter

[inspirit]

To protect against ABA issues it is sufficient to only update the counter in pop operation, isn't it?
Might be wrong though...

[skeeto]

That's true, we only need to count either pops or pushes, not both. In
order to re-use a pointer, it needs to be popped and pushed back on.
Right now I'm just using it as a state-change counter.