private void removeAllItems(Set<Item> items) {
		if (items != null) {
			// remove items with array loop to avoid concurrentModification exception
			// which will be thrown by the 'removeItem' method otherwise
			Item[] array = items.toArray(new Item[items.size()]);
			for (int i = 0; i < array.length; ++i)
				removeItem(array[i].getOwner(), array[i]);
		}
	}