Write a method that returns a string made up of all the letters from 'first' to 'last'. You may assume that 'first' and 'last' are both letters and that they are the same case (both upper-case or both lower-case). If 'first' and 'last' are the same letter, just return a string with that letter in it. If 'last' comes alphabetically before 'first' return an empty string.