code: @Override
    public double[][] processIds(DBIDs ids, Relation<? extends NumberVector> database) {
      return CovarianceMatrix.make(database, ids).destroyToPopulationMatrix();
    }
docs: Compute Covariance Matrix for a collection of database IDs.
  
  @param ids a collection of ids
  @param database the database used
  @return Covariance Matrix