public void TextNavigator_IsLetter_1()
        {
            const string  text      = "A1BC";
            TextNavigator navigator = new TextNavigator(text);

            Assert.IsTrue(navigator.IsLetter());
            navigator.ReadChar();
            Assert.IsFalse(navigator.IsLetter());
            navigator.ReadChar();
            Assert.IsTrue(navigator.IsLetter());
        }