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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 _advance_cluster_time(self, cluster_time):
 """Internal cluster time helper."""

        if self._cluster_time is None:
            self._cluster_time = cluster_time
        elif cluster_time is not None:
            if cluster_time["clusterTime"] > self._cluster_time["clusterTime"]:
                self._cluster_time = cluster_time