code: def variables(self) -> Set[str]:
          
          variables: Set[str] = set()
          for connection in self.__connections:
              variables.update(connection.variables)
          return variables
docs: A set of all different |Node.variable| values of the |Node|
          objects directly connected to the actual |Element| object.
  
          Suppose there is an element connected to five nodes, which (partly)
          represent different variables:
  
          >>> from hydpy import Element, Node
          >>> element = Element('Test',
          ...                   inlets=(Node('N1', 'X'), Node('N2', 'Y1')),
          ...                   outlets=(Node('N3', 'X'), Node('N4', 'Y2')),
          ...                   receivers=(Node('N5', 'X'), Node('N6', 'Y3')),
          ...                   senders=(Node('N7', 'X'), Node('N8', 'Y4')))
  
          Property |Element.variables| puts all the different variables of
          these nodes together:
  
          >>> sorted(element.variables)
          ['X', 'Y1', 'Y2', 'Y3', 'Y4']