public void ShouldNotThrowException_WhenSufficientFundsAreAvailable()
            {
                Account sut = new AccountBuilder().WithBalance(10m);

                Assert.DoesNotThrow(() => { sut.EnsureSufficientFundsAreAvailable(5m); });
            }
            public void ShouldThrowException_WhenInsufficientFundsAreAvailable()
            {
                Account sut = new AccountBuilder().WithBalance(10m);

                Assert.Throws <InvalidOperationException>(() => { sut.EnsureSufficientFundsAreAvailable(20m); });
            }