using System;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        // Iterate from a starting point until you encounter any elements defined as 

        // terminating elements

        var terminatingElements = new HashSet<int>{ 7, 9, 11 };

        // This will iterate from 1 until one of the terminating  elements is encountered (7)

        foreach(var x in CountUntilAny(1,terminatingElements))

        {

            Console.WriteLine(x);

        }

    }

    public static IEnumerable<int> CountUntilAny(int start, HashSet<int> earlyTerminationSet)

    {

        int curr = start;

        while (true)

        {

            if (earlyTerminationSet.Contains(curr))

            {

                // we've hit one of the ending values

                yield break;

            }

            yield return curr;

            if (curr == Int32.MaxValue)

            {

                // don't overflow if we get all the way to the end; just stop

                yield break;

            }

            curr++;

        }

    }

}