public ListItem<T> listsInList(ListItem<ListItem<T>> lsts) throws IllegalArgumentException
    {
        if (lsts == null)
        {
            throw new IllegalArgumentException("listsInList:  The list must not be null");
        }
        else if (lsts.next == null)
        {
            // only one list in our list of lists -> return the key
            return lsts.key;
        }
        else
        {
            // check if our first list has only one element
            if (lsts.key.next == null)
            {
                // yes : start recursion with the first element of the next list
                lsts.key.next = listInListRec(lsts.next, lsts.next.key);
            }
            else
            {
                // no : start recursion with the second element of the first list
                lsts.key.next = listInListRec(lsts, lsts.key.next);
            }
            return lsts.key;
        }

    }

    /**
     * Hilfsmethode f眉r {@link Aufgabe_1.A.A#listsInList(data.ListItem)}
     *
     * @param remainingLists
     * @param currentItem
     * @return the next ListItem
     */
    private ListItem<T> listInListRec(ListItem<ListItem<T>> remainingLists, ListItem<T> currentItem)
    {
        if (remainingLists.next == null && currentItem.next == null)
        {
            return currentItem;
        }
        else if (currentItem.next == null)
        {
            currentItem.next = listInListRec(remainingLists.next, remainingLists.next.key);
            return currentItem;
        }
        else
        {
            currentItem.next = listInListRec(remainingLists, currentItem.next);
            return currentItem;
        }
    }