def extra(name: str, desc: str) -> Callable:
    """
    Decorator for slave channel's "additional features" interface.

    Args:
        name (str): A human readable name for the function.
        desc (str): A short description and usage of it. Use
            ``{function_name}`` in place of the function name
            in the description.

    Returns:
        The decorated method.
    """

    def attr_dec(f):
        f.__setattr__("extra_fn", True)
        f.__setattr__("name", name)
        f.__setattr__("desc", desc)
        return f

    return attr_dec