class Program {
    // an ordinary function that is called by value
    public static void PrintStack<T>(Stack<T> s) {
        Stack<T> r = new Stack<T>();
        // pop and print
        while (s.Count != 0) {
            T elem = s.Pop();
            Console.Out.WriteLine(elem);
            r.Push(elem);
        }
        // put it all back
        while (r.Count != 0) {
            s.Push(r.Pop());
        }
    }

    // a common programming mistake: s is a copy of the
    // one the function was called with, so s = r just
    // reassigns s _local to this function_, so the change
    // does not persist
    public static void PrintStack2<T>(Stack<T> s) {
        Stack<T> r = new Stack<T>();
        // pop and print
        while (s.Count != 0) {
            T elem = s.Pop();
            Console.Out.WriteLine(elem);
            r.Push(elem);
        }
        s = r;
    }

    // This version actually _does_ work, though, because
    // C# lets you use call-by-reference if you want
    // (when using the ref keyword).
    public static void PrintStack3<T>(ref Stack<T> s) {
        Stack<T> r = new Stack<T>();
        // pop and print
        while (s.Count != 0) {
            T elem = s.Pop();
            Console.Out.WriteLine(elem);
            r.Push(elem);
        }
        s = r;
    }

    public static void Main(string[] args) {
        Stack<string> s = new Stack<string>();
        s.Push("class");
        s.Push("dear");
        s.Push("hello");

	// works great
	Console.Out.WriteLine("\nPrintStack (call-by-value semantics):");
        PrintStack(s);

	// does not work great
        // in fact, PrintStack2 empties
        // the stack. yuck.
	Console.Out.WriteLine("\nPrintStack 2 (call-by-value; broken though):");
        PrintStack2(s);
	Console.Out.WriteLine("Nothing printed here ->");
        PrintStack(s);
	Console.Out.WriteLine("<- Nothing printed here");

	// I need to restore the stack because
	// PrintStack2 mangled it
        s.Push("class");
        s.Push("dear");
        s.Push("hello");

	// but this does work
        // note that I have to 'opt in'
        // to call-by-reference both in the
        // function definition AND in the
        // function call
	Console.Out.WriteLine("\nPrintStack 3 (call-by-reference):");
	PrintStack3(ref s);
	PrintStack(s);
    }
}