code: def _calldata_fields(fields, exclude_fields, format_ids):
      
      if fields is None:
          # no fields specified by user
          # default to all standard fields plus all FORMAT fields in VCF header
          fields = config.STANDARD_CALLDATA_FIELDS + format_ids
      else:
          # fields specified by user
          for f in fields:
              # check if field is standard or defined in VCF header
              if (f not in config.STANDARD_CALLDATA_FIELDS and
                      f not in format_ids):
                  # support extracting FORMAT even if not declared in header,
                  # but warn...
                  print('WARNING: no definition found for field %s' % f,
                        file=sys.stderr)
      # process exclusions
      if exclude_fields is not None:
          fields = [f for f in fields if f not in exclude_fields]
      return tuple(fields)
docs: Utility function to determine which calldata (i.e., FORMAT) fields to
      extract.