code: private void addCohortStatsMultiInfoField(StudyEntry studyEntry, Map<String, Object> attributes) {
          if (studyEntry.getStats() == null || studyEntry.getStats().size() == 0) {
              return;
          }
          for (Map.Entry<String, VariantStats> entry : studyEntry.getStats().entrySet()) {
              String cohortName = entry.getKey();
              VariantStats stats = entry.getValue();
  
              if (cohortName.equals(StudyEntry.DEFAULT_COHORT)) {
                  cohortName = "";
                  int an = stats.getAltAlleleCount();
                  if (an >= 0) {
                      attributes.put(cohortName + VCFConstants.ALLELE_NUMBER_KEY, String.valueOf(an));
                  }
                  if (stats.getAltAlleleCount() >= 0) {
                      attributes.put(cohortName + VCFConstants.ALLELE_COUNT_KEY, String.valueOf(stats.getAltAlleleCount()));
                  }
              } else {
                  cohortName = cohortName + "_";
              }
              attributes.put(cohortName + VCFConstants.ALLELE_FREQUENCY_KEY, DECIMAL_FORMAT_7.format(stats.getAltAlleleFreq()));
          }
      }
docs: /*
  this function was moved to the parent class: VariantContextConverter
  private String getReferenceBase(String chromosome, Integer from, Integer to) {
  int length = to - from;
  if (length < 0) {
  throw new IllegalStateException(
  "Sequence length is negative: chromosome " + chromosome + " from " + from + " to " + to);
  }
  return StringUtils.repeat('N', length); // current return default base TODO load reference sequence
  }