public void Sum_Method()
        {
            Assert.Throws <ArgumentNullException>(() => IYandexMoneyButtonWidgetExtensions.Sum(null, 0));

            new YandexMoneyButtonWidget().Do(widget =>
            {
                Assert.True(ReferenceEquals(widget.Sum(1.0), widget));
                Assert.Equal((decimal)1.0, widget.Sum());
            });
        }