code: def check_extensions(extensions: Set[str], allow_multifile: bool = False):
      
      check_var(extensions, var_types=set, var_name='extensions')
  
      # -- check them one by one
      for ext in extensions:
          check_extension(ext, allow_multifile=allow_multifile)
docs: Utility method to check that all extensions in the provided set are valid
  
      :param extensions:
      :param allow_multifile:
      :return: