code: public void addDataSource(int groupno, DataSource datasource) {
      // the loader takes care of validation
      if (groupno == 0)
        datasources.add(datasource);
      else if (groupno == 1)
        group1.add(datasource);
      else if (groupno == 2)
        group2.add(datasource);
    }
docs: Adds a data source to the configuration. If in deduplication mode
  groupno == 0, otherwise it gives the number of the group to which
  the data source belongs.