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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 _start_nodes_sequentially(self, nodes):
 """ Start the nodes sequentially without forking. Return set of nodes that were actually started. """

        started_nodes = set()
        for node in copy(nodes):
            started = self._start_node(node)
            if started:
                started_nodes.add(node)
            # checkpoint cluster state
            self.repository.save_or_update(self)
        return started_nodes