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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 refresh(self):
 """Synchronize the local tree of Solr records for DataONE identifiers and queries with the reference tree."""

        if self._source_tree.cache_is_stale():
            self._source_tree.refresh()
            logging.info('Refreshing object tree')
            self._init_cache()
            self.sync_cache_with_source_tree()