docs: def focus(self, model):
          """Sets the passed model as focused element
  
          :param ModelMT model: The element to be focused
          """
          if model is None:
              del self.focus
              return
  
          self._check_model_types(model)
          self.add(model)
          focus_msg = FocusSignalMsg(model, self._focus)
          self._focus = model
          self._selected.add(model)
          self._selected = reduce_to_parent_states(self._selected)
          self.focus_signal.emit(focus_msg)