Macro ratio_def::units

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