def single_gen(msg):
    """Turn a single message into a plan

    If ``lambda x: yield x`` were valid Python, this would be equivalent.
    In Python 3.6 or 3.7 we might get lambda generators.

    Parameters
    ----------
    msg : Msg
        a single message

    Yields
    ------
    msg : Msg
        the input message
    """
    return (yield msg)
