using System;

using System.Collections;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        IEnumerable<char> s = "ABC";

        var l = new List<char>(s);

        Console.WriteLine(l[0]);

    }

        public IEnumerable<IEnumerable<T>> Combinations<T>(IEnumerable<T> iterable, int r)

        {

            int n = iterable.Count();

            int i, breakWhile;

​

            if (r > n)

                yield break;

​

            int[] range = Enumerable.Range(0, r).ToArray();

            int[] reversedRange = range.Reverse().ToArray();

​

            IEnumerable<T> firstOutput, output;

​

            firstOutput = range.Select(x => iterable.ElementAt(x)).ToArray();

​

            yield return firstOutput;

​

            while (true)

            {

                i = 0;

                breakWhile = -1;

​

                for (; i < range.Length; i++)

                {

                    if (range.ElementAt(reversedRange.ElementAt(i)) != reversedRange.ElementAt(i) + n - r)

                    {

                        breakWhile++;

                        break;

                    }

                }

​

                if (breakWhile < 0) break;

​

                range[reversedRange.ElementAt(i)] += 1;

​

                foreach (int j in Enumerable.Range(reversedRange.ElementAt(i) + 1, r - (reversedRange.ElementAt(i) + 1)))

                    range[j] = range[j - 1] + 1;

​

                output = range.Select(x => iterable.ElementAt(x)).ToArray();

​

                yield return output;

            }

        }

}