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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 reset_title(self, title, notebook_identifier):
 """Triggered whenever a notebook tab is switched in the left bar. Resets the title of the un-docked window to the format 'upper_open_tab / lower_open_tab' :param title: The name of the newly selected tab :param notebook: string taking one of two values 'upper' or 'lower' indicating which notebook was changed """

        current_title = self.get_top_widget().get_title()
        upper_title = current_title.split('/')[0].strip()
        lower_title = current_title.split('/')[1].strip()
        if notebook_identifier == 'upper':
            new_title = title + ' / ' + lower_title
        else:
            new_title = upper_title + ' / ' + title
        self['headerbar'].props.title = new_title