code: def elements(self) -> devicetools.Elements:
          
          elements = devicetools.Elements()
          for selection in self:
              elements += selection.elements
          return elements
docs: A |set| containing the |Node| objects of all handled
          |Selection| objects.
  
          >>> from hydpy import Selection, Selections
          >>> selections = Selections(
          ...     Selection('sel1', ['node1'], ['element1']),
          ...     Selection('sel2', ['node1'], ['element2', 'element3']))
          >>> selections.elements
          Elements("element1", "element2", "element3")