docs: def reference_creator_surnames(self, index):
          """Return as a list the surnames of the reference creators (locally defined)."""
          # TODO Not true, ex: ISBN 978-1-4398-3778-8. Return all creator types?
          # Academic books published as a collection of chapters contributed by
          # different authors have editors but not authors at the level of the
          # book (as opposed to the level of a chapter).
          creators = self.reference_data(index)["creators"]
          creator_types = [x["creatorType"] for x in creators]
          # 'name' (not split) might be used instead of 'firstName' and 'lastName'.
          try:
              if "author" in creator_types:
                  return [x["lastName"] for x in creators if x["creatorType"] == "author"]
              else:
                  return [x["lastName"] for x in creators]
          except KeyError:
              return []