code: public static boolean addExecutorServiceDecorator(String key, BiFunction<Scheduler, ScheduledExecutorService, ScheduledExecutorService> decorator) {
  		synchronized (DECORATORS) {
  			return DECORATORS.putIfAbsent(key, decorator) == null;
  		}
  	}
docs: Set up an additional {@link ScheduledExecutorService} decorator for a given key
  only if that key is not already present. Note that the {@link Factory}'s legacy
  {@link Factory#decorateExecutorService(String, Supplier)} method will always be
  invoked last after applying the decorators added via this method.
  <p>
  The decorator is a {@link BiFunction} taking the Scheduler and the backing
  {@link ScheduledExecutorService} as second argument. It returns the
  decorated {@link ScheduledExecutorService}.
  
  @param key the key under which to set up the decorator
  @param decorator the executor service decorator to add, if key not already present.
  @return true if the decorator was added, false if a decorator was already present
  for this key.
  @see #setExecutorServiceDecorator(String, BiFunction)
  @see #removeExecutorServiceDecorator(String)
  @see Schedulers#onScheduleHook(String, Function)