code: def router_elements(self):
          
          elements = devicetools.Elements()
          for router in self._router_numbers:
              element = self._get_routername(router)
              inlet = self._get_nodename(router)
              try:
                  outlet = self._get_nodename(self._up2down[router])
              except TypeError:
                  outlet = self.last_node
              elements += devicetools.Element(
                  element, inlets=inlet, outlets=outlet)
          return elements
docs: A |Elements| collection of all "routing" basins.
  
          (Only river basins with a upstream basin are assumed to route
          something to the downstream basin.)
  
          >>> from hydpy import RiverBasinNumbers2Selection
          >>> rbns2s = RiverBasinNumbers2Selection(
          ...                            (111, 113, 1129, 11269, 1125, 11261,
          ...                             11262, 1123, 1124, 1122, 1121))
  
          The following elements are properly connected to the required
          inlet and outlet nodes already:
  
          >>> for element in rbns2s.router_elements:
          ...     print(repr(element))
          Element("stream_1123",
                  inlets="node_1123",
                  outlets="node_1125")
          Element("stream_1125",
                  inlets="node_1125",
                  outlets="node_1129")
          Element("stream_11269",
                  inlets="node_11269",
                  outlets="node_1129")
          Element("stream_1129",
                  inlets="node_1129",
                  outlets="node_113")
          Element("stream_113",
                  inlets="node_113",
                  outlets="node_outlet")
  
          It is both possible to change the prefix names of the elements
          and nodes, as long as it results in a valid variable name (e.g.
          does not start with a number):
  
          >>> rbns2s.router_prefix = 'c_'
          >>> rbns2s.node_prefix = 'd_'
          >>> rbns2s.router_elements
          Elements("c_1123", "c_1125", "c_11269", "c_1129", "c_113")