You never take something back. A grow set for example only ever grows. A monotonic counter only ever grows. Even a set where you can insert and delete once is l

You never take something back. A grow set for example only ever grows. A monotonic counter only ever grows. Even a set where you can insert and delete once is like a set of counters that only ever grow (nothing: 0, added:1, removed:2). Forgetting for example is fine, but deletion is not.