Macro with_unknown

macro_rules! with_unknown {
    (
        $(
            $(#[$outer:meta])*
            $vis:vis enum $Enum:ident: $type:ty $([default = $dfl:expr])? {
                $(
                    $(#[$inner:meta])*
                    $enumerator:ident = $value:expr,
                )*
            }
        )*
    ) => { ... };
}