code: def collect_variables(self, selections) -> None:
          
          super().collect_variables(selections)
          self.insert_variables(self.device2base, self.basespecs, selections)
docs: Apply method |ChangeItem.collect_variables| of the base class
          |ChangeItem| and also apply method |ExchangeItem.insert_variables|
          of class |ExchangeItem| to collect the relevant base variables
          handled by the devices of the given |Selections| object.
  
          >>> from hydpy.core.examples import prepare_full_example_2
          >>> hp, pub, TestIO = prepare_full_example_2()
          >>> from hydpy import AddItem
          >>> item = AddItem(
          ...     'alpha', 'hland_v1', 'control.sfcf', 'control.rfcf', 0)
          >>> item.collect_variables(pub.selections)
          >>> land_dill = hp.elements.land_dill
          >>> control = land_dill.model.parameters.control
          >>> item.device2target[land_dill] is control.sfcf
          True
          >>> item.device2base[land_dill] is control.rfcf
          True
          >>> for device in sorted(item.device2base, key=lambda x: x.name):
          ...     print(device)
          land_dill
          land_lahn_1
          land_lahn_2
          land_lahn_3