code: def endnodes(self):
          
          endnodes = devicetools.Nodes()
          for node in self.nodes:
              for element in node.exits:
                  if ((element in self.elements) and
                          (node not in element.receivers)):
                      break
              else:
                  endnodes += node
          return endnodes
docs: |Nodes| object containing all |Node| objects currently handled by
          the |HydPy| object which define a downstream end point of a network.