public static void WhenPassingInRandomLettersReturnTheCorrectSequence()
        {
            ISequenceGenerator generator = new EncryptingSequenceGenerator();

            Assert.Equal("vwxyzabcdefghijklmnopqrstu", generator.GenerateSequence("V"));
            Assert.Equal("defghijklmnopqrstuvwxyzabc", generator.GenerateSequence("d"));
            Assert.Equal("rstuvwxyzabcdefghijklmnopq", generator.GenerateSequence("R"));
        }
        public static void WhenPassingInACharacterThatDoesntExistThrowOutOfRangeException()
        {
            ISequenceGenerator generator = new EncryptingSequenceGenerator();

            Assert.Throws <ArgumentOutOfRangeException>(() => generator.GenerateSequence("@"));
        }