Macro quantity

macro_rules! quantity {
    (        
        $(#[$quantity_attr:meta])* quantity: $quantity:ident; $description:expr;
        $(#[$dim_attr:meta])* dimension: $system:ident[$($dimension:expr),+];
        $(kind: $kind:ty;)?
        units {            
            $($(#[$unit_attr:meta])* @$unit:ident: $conversion:expr; $abbreviation:literal,
                $singular:literal, $plural:literal;)+
        }
    ) => { ... };
}