code: def tone_marks():
      
      return RegexBuilder(
          pattern_args=symbols.TONE_MARKS,
          pattern_func=lambda x: u"(?<={}).".format(x)).regex
docs: Keep tone-modifying punctuation by matching following character.
  
      Assumes the `tone_marks` pre-processor was run for cases where there might
      not be any space after a tone-modifying punctuation mark.