code: public ArrayList<Integer> trackerTiers() {
          int_vector v = p.get_tracker_tiers();
          int size = (int) v.size();
          ArrayList<Integer> l = new ArrayList<>();
  
          for (int i = 0; i < size; i++) {
              l.add(v.get(i));
          }
  
          return l;
      }
docs: The tiers the URLs in {@link #trackers()} belong to. Trackers belonging to
  different tiers may be treated differently, as defined by the multi
  tracker extension. This is optional, if not specified trackers are
  assumed to be part of tier 0, or whichever the last tier was as
  iterating over the trackers.
  
  @return the list of trackers tiers