code: def add_suffix(string, suffix):
      
      if string[-len(suffix):] != suffix:
          return string + suffix
      else:
          return string
docs: Adds a suffix to a string, if the string does not already have that suffix.
  
      :param string: the string that should have a suffix added to it
      :param suffix: the suffix to be added to the string
      :return: the string with the suffix added, if it does not already end in
          the suffix. Otherwise, it returns the original string.