code: def _token_to_subwords(self, token):
      
      subwords = []
  
      start = 0
      while start < len(token):
        subword = None
        for end in range(
            min(len(token), start + self._max_subword_len), start, -1):
          candidate = token[start:end]
          if (candidate in self._subword_to_id or
              candidate == _UNDERSCORE_REPLACEMENT):
            subword = candidate
            subwords.append(subword)
            start = end
            break
        # No subword match found. Consume a single (unicode) character.
        if subword is None:
          subwords.append(token[start])
          start += 1
  
      return subwords
docs: Greedily split token into subwords.