docs: public void purge(Instant limit) {
  		Instant firstKeeper = values.floorKey(limit);
  		if (firstKeeper == null) return;
  		Set<Instant> purgables = values.headMap(firstKeeper).keySet();
  		if (purgables == null) return;
  		for (Instant purgable : purgables) {
  			values.remove(purgable);
  		}
  	}