code: def _index_unknown_and_reserved_tokens(self, unknown_token, reserved_tokens):
          
  
          self._unknown_token = unknown_token
          # Thus, constants.UNKNOWN_IDX must be 0.
          self._idx_to_token = [unknown_token]
  
          if reserved_tokens is None:
              self._reserved_tokens = None
          else:
              self._reserved_tokens = reserved_tokens[:]
              self._idx_to_token.extend(reserved_tokens)
  
          self._token_to_idx = {token: idx for idx, token in enumerate(self._idx_to_token)}
docs: Indexes unknown and reserved tokens.