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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 _prepare_persistence_engine(self):
 """Load the specified persistence engine, or the default if none is set. """

        if self._persistence_engine:
            return

        persistence_engine = self._options.get('persistence_engine')
        if persistence_engine:
            self._persistence_engine = path_to_reference(persistence_engine)
            return

        from furious.config import get_default_persistence_engine

        self._persistence_engine = get_default_persistence_engine()