rds_iter()
        :param record_details: Details about the record returned by
            get_build_details
        :param base_content: If get_build_details returns a compression_parent,
            you must return a base_content here, else use None
        :param copy_base_content: When building from the base_content, decide
            you can either copy it and return a new object, or modify it in
            place.
        :return: (content, delta) A Content object and possibly a line-delta,
            delta may be None
        r›