class StrictSubscriber extends AtomicInteger implements FlowableSubscriber<T>, Subscription {

    @Override
    public void onSubscribe(Subscription s) {
        if (once.compareAndSet(false, true)) {

            actual.onSubscribe(this);

            SubscriptionHelper.deferredSetOnce(this.s, requested, s);
        } else {
            s.cancel();
            cancel();
            onError(new IllegalStateException("§2.12 violated: onSubscribe must be called at most once"));
        }
    }

    public  StrictSubscriber(Subscriber<? super T> actual);

}