public void Test_longest_string()
        {
            Example_class example = new Example_class();

            // make sure it returns longest string
            Assert.AreEqual("BBB",
                            example.Longest_string("AA", "BBB"));

            // make sure that even-case result is the first arg
            Assert.AreEqual("Hey",
                            example.Longest_string("Hey", "Bro"));
        }