docs: def store(self, transient_file, persistent_file):
          '''Makes PersistentFile from TransientFile'''
          #for i in range(5):
          #    persistent_file = PersistentFile(self.persistent_root,
          #                                     persistent_name, self)
          #    if not os.path.exists(persistent_file.path):
          #        break
          #else:
          #    raise Exception('Unable to find free file name')
          dirname = os.path.dirname(persistent_file.path)
          if not os.path.isdir(dirname):
              os.makedirs(dirname)
          os.rename(transient_file.path, persistent_file.path)
          return persistent_file