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