code: def update_devices(self, selection=None):
          
          if selection is not None:
              self.nodes = selection.nodes
              self.elements = selection.elements
          self._update_deviceorder()
docs: Determines the order, in which the |Node| and |Element| objects
          currently handled by the |HydPy| objects need to be processed during
          a simulation time step.  Optionally, a |Selection| object for defining
          new |Node| and |Element| objects can be passed.