private Action<T> pendingAddedAction;
    private EventSubscriptionVerifier<T> subscriptionVerifier = type -> false;

    protected int modCount;

    List<Element<T>> getInserted() {
        return inserted;
    }

    @Override
    public boolean isEmpty() {
        for (Element<T> element : inserted) {
            if (element.size() != 0) {
                return false;