code: def add_index_argument(cls, group):
          
          prefix = cls.argument_prefix
  
          group.add_argument(
              '--%s-index' % prefix, action="store",
              dest="%s_index" % prefix,
              help=("Name of the %s root markdown file, can be None" % (
                  cls.extension_name)))
docs: Subclasses may call this to add an index argument.
  
          Args:
              group: arparse.ArgumentGroup, the extension argument group
              prefix: str, arguments have to be namespaced