    def add_tokens_to_namespace(self, tokens: List[str], namespace: str = "tokens") -> List[int]:
        """
        Adds `tokens` to the index, if they are not already present.  Either way, we return the
        indices of the tokens in the order that they were given.
        """
        return [self.add_token_to_namespace(token, namespace) for token in tokens]
Adds `tokens` to the index, if they are not already present.  Either way, we return the
indices of the tokens in the order that they were given.
allennlp/data/vocabulary.py