How to use the sniffio.current_async_library_cvar.set function in sniffio

try:
        asynclib_name = sniffio.current_async_library()
    except sniffio.AsyncLibraryNotFoundError:
        pass
    else:
        raise RuntimeError(f'Already running {asynclib_name} in this thread')

    try:
        asynclib = import_module(f'{__name__}._backends._{backend}')
    except ImportError as exc:
        raise LookupError(f'No such backend: {backend}') from exc

    token = None
    if sniffio.current_async_library_cvar.get(None) is None:
        # Since we're in control of the event loop, we can cache the name of the async library
        token = sniffio.current_async_library_cvar.set(backend)

    try:
        backend_options = backend_options or {}
        return asynclib.run(func, *args, **backend_options)  # type: ignore
    finally:
        if token:
            sniffio.current_async_library_cvar.reset(token)