@classmethod<EOL><INDENT>def add_index_argument(cls, group):<DEDENT>
prefix = cls.argument_prefix<EOL>group.add_argument(<EOL>'<STR_LIT>' % prefix, action="<STR_LIT:store>",<EOL>dest="<STR_LIT>" % prefix,<EOL>help=("<STR_LIT>" % (<EOL>cls.extension_name)))<EOL>
Subclasses may call this to add an index argument.

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