code: public Collection<DataSource> getDataSources(int groupno) {
      if (groupno == 1)
        return group1;
      else if (groupno == 2)
        return group2;
      else
        throw new DukeConfigException("Invalid group number: " + groupno);
    }
docs: Returns the data sources belonging to a particular group of data
  sources. Data sources are grouped in record linkage mode, but not
  in deduplication mode, so only use this method in record linkage
  mode.