code: private static Set<String> determineVcfInfoFlagFields(VcfMeta vcfMeta) {
      return stream(vcfMeta.getInfoMeta())
          .filter(vcfInfoMeta -> vcfInfoMeta.getType().equals(VcfMetaInfo.Type.FLAG))
          .map(VcfMetaInfo::getId)
          .collect(toSet());
    }
docs: Returns a set of all possible VCF info fields of type 'Flag'
  
  @param vcfMeta VCF metadata
  @return Set of VCF info fields of type 'Flag'