docs: def _nodes(self):
          """
          Returns the list of nodes present in the network
  
          Examples
          --------
          >>> from pgmpy.models import DynamicBayesianNetwork as DBN
          >>> dbn = DBN()
          >>> dbn.add_nodes_from(['A', 'B', 'C'])
          >>> sorted(dbn._nodes())
          ['B', 'A', 'C']
          """
          return list(set([node for node, timeslice in
                           super(DynamicBayesianNetwork, self).nodes()]))