code: @SuppressWarnings("unchecked")
    public <C extends CDACallback<SynchronizedSpace>> C fetch(C callback) {
      return (C) Callbacks.subscribeAsync(observe(), callback, client);
    }
docs: Invokes the request to sync (asynchronously) with the provided {@code callback}.
  
  @param callback callback.
  @param <C>      callback type.
  @return the given callback instance.