public static List<List<T>> ShowAllCombinations<T>(IList<T> arr, List<List<T>> list = null, List<T> current = null)
        {
            if (list == null) list = new List<List<T>>();
            if (current == null) current = new List<T>();
            if (arr.Count == 0) 
            {
                list.Add(current);
                return list;
            }
            for (int i = 0; i < arr.Count; i++) 
            {
                List<T> lst = new List<T>(arr);
                lst.RemoveAt(i);
                var newlst = new List<T>(current);
                newlst.Add(arr[i]);
                ShowAllCombinations(lst, list, newlst);
            }
            return list;
        }