Returns:
            ReadInstruction instance.
        """
        spec = str(spec)  # Need to convert to str in case of NamedSplit instance.
        subs = _ADDITION_SEP_RE.split(spec)
        if not subs:
            raise ValueError(f"No instructions could be built out of {spec}")
        instruction = _str_to_read_instruction(subs[0])
        return sum((_str_to_read_instruction(sub) for sub in subs[1:]), instruction)