Function seeded_waypoints 

fn seeded_waypoints<'a>(
    drawing: &'a Drawing<'_>,
    seeding: &'a Seeding<'_>,
    id: RouteId,
) -> impl Iterator<Item = GridPoint> + 'a

Where a lattice holds id’s waypoints this pass: offset with a group drag it rides entirely inside, and without the ones a preview trims or prunes.