using System;

public class Program

{

    static void test(bool? val = null)

    {

        if (val == true)

            Console.WriteLine("yes");

        else if (val == false)

            Console.WriteLine("no");

        else

            Console.WriteLine("not sure");

    }

    public static void Main()

    {

        test();

        test(true);

        test(false);

        test(null);

    }

}