Function proj2 

fn proj2<E, Ix: IndexType>(
    (row_index, row): (usize, &Vec<WSuc<E, Ix>>),
) -> Map<Zip<Enumerate<Iter<'_, WSuc<E, Ix>>>, Repeat<Ix>>, fn(((usize, &'_ WSuc<E, Ix>), Ix)) -> EdgeReference<'_, E, Ix>>