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

            Assert.IsTrue(x.Size() == 0);
            x.Push(10);
            x.Push(20);
            x.Push(30);
            x.Push(40);
            x.Subtract();
            Assert.IsTrue(x.Top() == 10);
        }