public void SubtractShouldReturnTheCalculatorInstance()
        {
            Calculator calculator2 = calculator.Subtract(four);

            Assert.AreSame(calculator, calculator2);
        }
        public void DivideShouldReturnTheCalculatorInstance()
        {
            Calculator calculator2 = calculator.Divide(seven);

            Assert.AreSame(calculator, calculator2);
        }
        public void ResetShouldReturnTheCalculatorInstance()
        {
            Calculator calculator2 = calculator.Reset();

            Assert.AreSame(calculator, calculator2);
        }