code: private ArrayDBIDs initialSet(DBIDs sampleSet, int k, Random random) {
      return DBIDUtil.ensureArray(DBIDUtil.randomSample(sampleSet, k, random));
    }
docs: Returns a set of k elements from the specified sample set.
  
  @param sampleSet the sample set
  @param k the number of samples to be returned
  @param random random number generator
  @return a set of k elements from the specified sample set