docs: public GenomicsDataSource<Read, ReadGroupSet, Reference> get(String rootUrl) {
      Data<Read, ReadGroupSet, Reference> data = dataSources.get(rootUrl);
      if (data == null) {
        data = new Data<Read, ReadGroupSet, Reference>(new Settings(), null);
        dataSources.put(rootUrl, data);
      }
      if (data.dataSource == null) {
        data.dataSource = makeDataSource(rootUrl, data.settings);
      }
      return data.dataSource;
    }