controller.py

    :return Deferred: A ``Deferred`` which fires with the result of
        ``between``.
    """
    yield first()
    try:
        result = yield between()
    except GeneratorExit:
        raise
    except:
        info = exc_info()
        yield last()
        raise info[0], info[1], info[2]
    else:
        yield last()
        returnValue(result)