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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 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)