public void UnitFormatter_When_InputIs2Digits_Then_ResultIsInvalidDigitsException()
        {
            // arrange
            var formatter = new UnitFormatter();

            // act
            // assert
            Assert.Throws<InvalidDigitsException>(() => formatter.Format(10));
        }
        public void UnitFormatter_When_InputIs1Digit_Then_ResultIsNoException()
        {
            // arrange
            var formatter = new UnitFormatter();

            // act
            // assert
            Assert.DoesNotThrow(() => formatter.Format(9));
        }
        public void UnitFormatter_When_InputIs9_Then_ResultIsNine()
        {
            // arrange
            var formatter = new UnitFormatter();

            // act
            var value = formatter.Format(9);

            // assert
            Assert.That(value, Is.EqualTo("nine"));
        }