using System;

public class Test
{
	public static void Main()
	{
        	Console.WriteLine(f(new Nullable<bool>()));
        	Console.WriteLine(f(new Nullable<bool>(true)));
        	Console.WriteLine(f(new Nullable<bool>(false)));
	}
        private static string f(bool? arg)
        {
            switch (arg)
            {
                case true: return "true";
                case false: return "false";
                case null: return "null";
                default: return "???";
            }
        }
 
}