Macro metron::def_exp_scale

macro_rules! def_exp_scale {
    (
        $(
            $vis:vis $name:ident[
            $(
                $base:ty =^ $exp:ty ,
            )*
            ]
            $(
                : default $base_default:ty[$b:ty, $e:ty]
            )?
            ,
        )*
    ) => { ... };
}