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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 filename_for(self, subpath):
 """ Returns the relative filename for the specified subpath, or the root filename if subpath is None. Raises werkzeug.exceptions.NotFound if the resulting path would fall out of the root directory. """

        try:
            filename = self.readme_for(subpath)
            return os.path.relpath(filename, self.root_directory)
        except ReadmeNotFoundError:
            return None