public void TestSecondTop()
        {
            IFancyCalculator x = new FancyCalculator();

            x.Push(25);
            x.Push(10);
            x.Push(15);
            Assert.IsTrue(x.SecondTop() == 10);
        }
        public void TestSecondTopFailureToRespond()
        {
            IFancyCalculator x = new FancyCalculator();

            Assert.IsNull(x.SecondTop());
        }