public void EnteringTheEqualSignDisplaysTheResult()
        {
            sut = sut.EnterDigit('3');
            sut = sut.EnterDigit('3');
            sut = sut.EnterEqual();

            Assert.AreEqual("55", sut.Display);
        }
        public void FurtherOperatorsExecuteThePreviousCalculation()
        {
            sut = sut.EnterDigit('3');
            sut = sut.EnterDigit('3');
            sut = sut.EnterOperator(new PlusOperator());
            sut = sut.EnterDigit('2');
            sut = sut.EnterDigit('2');
            sut = sut.EnterEqual();

            Assert.AreEqual("77", sut.Display);
        }