docs: 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()