docs: def get_from_archive(self, feed_id):
          """
          Retrieves feed that was persisted as .xml file by its id (= filename)
          Note: No check on feed validity. file content is assumed correct
          :param feed_id:
          :return: the atom feed as string
          """
          file_path = self.feed_repository + '/' + str(feed_id) + '.xml'
          if not os.path.isfile(file_path):
              raise FeedArchiveNotFound()
          with open(file_path, 'r') as rec_file:
              return rec_file.read()