code: def add_token_to_namespace(self, token: str, namespace: str = 'tokens') -> int:
          
          if not isinstance(token, str):
              raise ValueError("Vocabulary tokens must be strings, or saving and loading will break."
                               "  Got %s (with type %s)" % (repr(token), type(token)))
          if token not in self._token_to_index[namespace]:
              index = len(self._token_to_index[namespace])
              self._token_to_index[namespace][token] = index
              self._index_to_token[namespace][index] = token
              return index
          else:
              return self._token_to_index[namespace][token]
docs: Adds ``token`` to the index, if it is not already present.  Either way, we return the index of
          the token.