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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 fix_relative_url(self, publish_type, rel_url):
 """ Fix post or page relative url to a standard, uniform format. :param publish_type: publish type ('post' or 'page') :param rel_url: relative url to fix :return: tuple(fixed relative url or file path if exists else None, file exists or not) :raise ValueError: unknown publish type """

        if publish_type == 'post':
            return self.fix_post_relative_url(rel_url), False
        elif publish_type == 'page':
            return self.fix_page_relative_url(rel_url)
        else:
            raise ValueError(
                'Publish type "{}" is not supported'.format(publish_type))