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

            new YandexMoneyPaymentFormWidget().Do(widget =>
            {
                Assert.True(ReferenceEquals(widget.Sum(1.0), widget));
                Assert.Equal((decimal)1.0, widget.Sum());
            });
        }
        public void Text_Method()
        {
            Assert.Throws <ArgumentNullException>(() => IYandexMoneyPaymentFormWidgetExtensions.Text(null, YandexMoneyPaymentFormText.Pay));

            new YandexMoneyPaymentFormWidget().Do(widget =>
            {
                Assert.True(ReferenceEquals(widget.Text(YandexMoneyPaymentFormText.Pay), widget));
                Assert.Equal(1, widget.Text());
            });
            new YandexMoneyPaymentFormWidget().Do(widget => Assert.Equal(2, widget.Text(YandexMoneyPaymentFormText.Buy).Text()));
            new YandexMoneyPaymentFormWidget().Do(widget => Assert.Equal(3, widget.Text(YandexMoneyPaymentFormText.Transfer).Text()));
            new YandexMoneyPaymentFormWidget().Do(widget => Assert.Equal(4, widget.Text(YandexMoneyPaymentFormText.Give).Text()));
        }