class FutureItems {

  public void add(final T item) {
    final UInt64 slot = slotFunction.apply(item);
    LOG.trace("Save future item at slot {} for later import: {}", slot, item);
    queuedFutureItems.computeIfAbsent(slot, key -> createNewSet()).add(item);
  }

  public  FutureItems(final Function<T, UInt64> slotFunction);

}