code: private static void unregisterIfPossible(final ExecutorService executorService) {
  		if (executorService instanceof ScheduledExecutorService) {
  			SCHEDULERS.remove(executorService);
  		} else {
  			BACKGROUND_EXECUTORS.remove(executorService);
  		}
  	}
docs: Unregisters the given {@code ExecutorService} if it is an instance of {@code ScheduledExecutorService} from the
  list of registered schedulers.
  
  @param executorService
  a possible scheduler