langchain/document_loaders/url.py/UnstructuredURLLoader/_validate_mode
class UnstructuredURLLoader:    def _validate_mode(self, mode: str) -> None:
        _valid_modes = {"single", "elements"}
        if mode not in _valid_modes:
            raise ValueError(
                f"Got {mode} for `mode`, but should be one of `{_valid_modes}`"
            )