mod[0]
    -> [0]cj1;

mod[1]
    -> [1]cj1;

cj1 = cross_join()
    -> [0]cj2;

mod[2]
    -> [1]cj2;

cj2 = cross_join()
    -> map(|((a, b), c)| (a, b, c))
    -> mod;