def stream_results_optional(
    f: Callable[..., Optional[PagedResult[TResult, TToken]]], *args, **kwargs
) -> Optional[Iterable[TResult]]:
    """Like stream_results(), but for functions ``f`` that return an Optional.

    """
    if "page_token" in kwargs:
        raise TypeError('stream_results has no argument "page_token".')
    page_token = None
    for i in itertools.count():
        page_result = f(*args, page_token=page_token, **kwargs)
        if page_result is None:
            if page == 0:
                return None
            else:
                raise ValueError(
                    f"First iteration(s) of {f!r} return non-None, but iteration "
                    f"{i} (with page_token={page_token!r}) returned None."
                )
        yield from page_result.results
        page_token = page_result.next_page_token
        if page_token is None:
            break