code: public static ModifiableDBIDs randomSample(DBIDs source, int k, RandomFactory rnd) {
      return randomSample(source, k, rnd.getSingleThreadedRandom());
    }
docs: Produce a random sample of the given DBIDs.
  
  @param source Original DBIDs, no duplicates allowed
  @param k k Parameter
  @param rnd Random generator
  @return new DBIDs