docs: def _preprocess_chars(self, chars):
          '''
          Performs string preprocessing before the main conversion algorithm
          is used. Simple string replacements (for example, fullwidth rōmaji
          to regular rōmaji) are performed at this point.
          '''
          chars = self._normalize_dakuten(chars)
          chars = self._process_repeaters(chars)
          chars = self._perform_replacements(chars)
  
          return chars