*
     * @throws IllegalStateException if an id has already been loaded.
     */
    public long createId() {
        long newId = sequence.updateAndGet(it -> it == USED_ASSIGNED_ID_MARKER ? it : it + 1);
        if (newId == USED_ASSIGNED_ID_MARKER) {
            throw new IllegalStateException("Cannot create a new id after one has been loaded");
        }