private static void TestStack()
{
var stack = new Stack <int>();
stack.Push(6);
stack.Push(5);
stack.Push(4);
var stack1 = new Stack <int>();
stack1.Push(3);
stack1.Push(2);
stack1.Push(1);
Console.WriteLine(stack.Pop());
Console.WriteLine(stack.Peek());
stack.AddAll(stack1);
Output(stack);
Console.WriteLine("Complete.");
}