Macro units

macro_rules! units {
    (
        $(#[$blanket_meta:meta])*
        $blanket:ident($($super:tt)*);
        #[$derive:meta] $base:ty:
        $(
            $(#[$meta:meta])*
            $tys:ident;
        )*
        $(
            <$a:path> * <$b:path> = $c:path;
        )*
    ) => { ... };
}