using System;

using System.Linq;

using System.Collections.Generic;

using System.Diagnostics;

public class Program

{

    public static IEnumerable<int> Foo()

    {

        Console.WriteLine(new StackTrace());

        yield return 1;

    }

    public static void Main()

    {

        IEnumerable<int> c = Foo();

        for (int i = 0; i < 20; i += 1)

            c = new[] {2}.Concat(c);

        foreach(var i in c) 

            Console.WriteLine(i);

    }

}