docs: def missing_optional_tagfiles(self):
          """
          From v0.97 we need to validate any tagfiles listed
          in the optional tagmanifest(s). As there is no mandatory
          directory structure for additional tagfiles we can
          only check for entries with missing files (not missing
          entries for existing files).
          """
          for tagfilepath in list(self.tagfile_entries().keys()):
              if not os.path.isfile(os.path.join(self.path, tagfilepath)):
                  yield tagfilepath