docs: private static void shrinkHeap(TreeSet<DoubleDBIDPair> hotset, int k) {
      // drop duplicates
      ModifiableDBIDs seenids = DBIDUtil.newHashSet(2 * k);
      int cnt = 0;
      for(Iterator<DoubleDBIDPair> i = hotset.iterator(); i.hasNext();) {
        DoubleDBIDPair p = i.next();
        if(cnt > k || seenids.contains(p)) {
          i.remove();
        }
        else {
          seenids.add(p);
          cnt++;
        }
      }
    }