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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 __remove_obsolete_metadata(self):
 """ Removes obsolete entries from the metadata of all stored routines. """

        clean = {}
        for key, _ in self._source_file_names.items():
            if key in self._pystratum_metadata:
                clean[key] = self._pystratum_metadata[key]

        self._pystratum_metadata = clean