code: def create_prefix_dir(nb_file, fmt):
      
      if 'prefix' in fmt:
          nb_dir = os.path.dirname(nb_file) + os.path.sep
          if not os.path.isdir(nb_dir):
              logging.log(logging.WARNING, "[jupytext] creating missing directory %s", nb_dir)
              os.makedirs(nb_dir)
docs: Create directory if fmt has a prefix