def add_index_argument(cls, group):
        """
        Subclasses may call this to add an index argument.

        Args:
            group: arparse.ArgumentGroup, the extension argument group
            prefix: str, arguments have to be namespaced
        """
        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)))
Subclasses may call this to add an index argument.

        Args:
            group: arparse.ArgumentGroup, the extension argument group
            prefix: str, arguments have to be namespaced