code: public static boolean validate(@Nullable Subscription current, Subscription next) {
  		Objects.requireNonNull(next, "Subscription cannot be null");
  		if (current != null) {
  			next.cancel();
  			//reportSubscriptionSet();
  			return false;
  		}
  
  		return true;
  	}
docs: Check Subscription current state and cancel new Subscription if current is set,
  or return true if ready to subscribe.
  
  @param current current Subscription, expected to be null
  @param next new Subscription
  @return true if Subscription can be used