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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 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)))