Macro csv_lazy_slots 

macro_rules! csv_lazy_slots {
    (
        categories {
            $(
                $category_slot:ident, $category_reader:ident: $category_value:ty
                    => $category_transform:path, $category_key:expr
            ),* $(,)?
        }
        grouped {
            $(
                $grouped_slot:ident, $grouped_reader:ident: $grouped_value:ty
                    => $grouped_transform:path, $grouped_key:expr
            ),* $(,)?
        }
        slots {
            $( $slot:ident: $value:ty ),* $(,)?
        }
    ) => { ... };
}