code: public static <F> boolean replace(AtomicReferenceFieldUpdater<F, Subscription> field,
  			F instance,
  			Subscription s) {
  		for (; ; ) {
  			Subscription a = field.get(instance);
  			if (a == CancelledSubscription.INSTANCE) {
  				s.cancel();
  				return false;
  			}
  			if (field.compareAndSet(instance, a, s)) {
  				return true;
  			}
  		}
  	}
docs: A generic utility to atomically replace a subscription or cancel the replacement
  if the current subscription is marked as already cancelled (as in
  {@link #cancelledSubscription()}).
  
  @param field The Atomic container
  @param instance the instance reference
  @param s the subscription
  @param <F> the instance type
  
  @return true if replaced