Function product_len 

fn product_len(lists: &[Vec<(ItemId, String)>]) -> usize

The number of axis lines the Cartesian product of lists produces: the product of each list’s length (empty lists -> 1, the scalar axis; any empty list -> 0, no lines). Mirrors axis_tuples(..).len() but is O(k) in the number of categories, never materializing the product.