static void TestInterfaceExercise3()
{
ListInt ls = new NodeInt(10, new NodeInt(4, new NodeInt(5, new NodeInt(3, new EmptyInt()))));
ListInt lsFiltered = ls.Filter((x) => x % 2 == 0);
lsFiltered.Iter((x) => Console.WriteLine(x));
// Console.WriteLine (ls.Length);
}
public void Iter(Action <int> f)
{
f(head);
tail.Iter(f);
}