docs: 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