public static <E> Collection<List<E>> selectExactly(List<E> original, int nb) {
		if (nb < 0) {
			throw new IllegalArgumentException();
		}
		if (nb == 0) {
			return Collections.emptyList();
		}
		if (nb == 1) {
			final List<List<E>> result = new ArrayList<List<E>>();
			for (E element : original) {
				result.add(Collections.singletonList(element));
			}
			return result;

		}
		if (nb > original.size()) {
			return Collections.emptyList();
		}
		if (nb == original.size()) {
			return Collections.singletonList(original);
		}
		final List<List<E>> result = new ArrayList<List<E>>();

		for (List<E> subList : selectExactly(original.subList(1, original.size()), nb - 1)) {
			final List<E> newList = new ArrayList<E>();
			newList.add(original.get(0));
			newList.addAll(subList);
			result.add(Collections.unmodifiableList(newList));
		}
		result.addAll(selectExactly(original.subList(1, original.size()), nb));

		return Collections.unmodifiableList(result);
	}