code: def register_element(self, dev_name, idx=None):
          
          if dev_name not in self.devices:
              logger.error(
                  'Device {} missing. call add_device before adding elements'.
                  format(dev_name))
              return
          group_name = self.system.__dict__[dev_name]._group
          if idx is None:  # "if not idx" will fail for idx==0.0
              idx = dev_name + '_' + str(len(self.group[group_name].keys()))
          self.group[group_name][idx] = dev_name
          return idx
docs: Register a device element to the group list
  
          Parameters
          ----------
          dev_name : str
              model name
          idx : str
              element idx
  
          Returns
          -------
          str
              assigned idx