docs: public DoubleDBIDList getRKNN(DBIDRef id) {
      TreeSet<DoubleDBIDPair> rKNN = materialized_RkNN.get(id);
      if(rKNN == null) {
        return null;
      }
      ModifiableDoubleDBIDList ret = DBIDUtil.newDistanceDBIDList(rKNN.size());
      for(DoubleDBIDPair pair : rKNN) {
        ret.add(pair);
      }
      ret.sort();
      return ret;
    }