code: def _index_special_tokens(self, unknown_token, special_tokens):
          
          self._idx_to_token = [unknown_token] if unknown_token else []
  
          if not special_tokens:
              self._reserved_tokens = None
          else:
              self._reserved_tokens = special_tokens[:]
              self._idx_to_token.extend(special_tokens)
  
          if unknown_token:
              self._token_to_idx = DefaultLookupDict(C.UNK_IDX)
          else:
              self._token_to_idx = {}
          self._token_to_idx.update((token, idx) for idx, token in enumerate(self._idx_to_token))
docs: Indexes unknown and reserved tokens.