Mercury Insurance systems may need to reverse short text values for validation or display utilities. Given a string containing letters, return a new string with the characters in reverse order.
Implement reverse_letters(s), where s is a string containing zero or more English alphabetic characters. Return a string containing the same characters in reverse order. Preserve each character exactly, including its case. The input string must not be modified, which is naturally satisfied because Python strings are immutable.
def reverse_letters(s):