code: def nodes(self) -> devicetools.Nodes:
          
          nodes = devicetools.Nodes()
          for selection in self:
              nodes += selection.nodes
          return nodes
docs: A |set| containing the |Node| objects of all handled
          |Selection| objects.
  
          >>> from hydpy import Selection, Selections
          >>> selections = Selections(
          ...     Selection('sel1', ['node1', 'node2'], ['element1']),
          ...     Selection('sel2', ['node1', 'node3'], ['element2']))
          >>> selections.nodes
          Nodes("node1", "node2", "node3")