docs: def reference_extra_field(self, field, index):
          """Return the value of the field in 'extra', otherwise ''."""
          ref_data = self.reference_data(index)
          extra_fields = ref_data["extra"].split("\n")
          field_id = field + ":"
          matched = next((x for x in extra_fields if x.startswith(field_id)), None)
          if matched:
              return matched.replace(field_id, "", 1).strip()
          else:
              return ""