docs: def _find_base_version_ids(self, symbol, version_ids):
          """
          Return all base_version_ids for a symbol that are not bases of version_ids
          """
          cursor = self._versions.find({'symbol': symbol,
                                        '_id': {'$nin': version_ids},
                                        'base_version_id': {'$exists': True},
                                        },
                                       projection={'base_version_id': 1})
          return [version["base_version_id"] for version in cursor]