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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 _prepare_graph_terms(self, default_screen):
 """Helper for to_graph and to_execution_plan."""

        columns = self.columns.copy()
        screen = self.screen
        if screen is None:
            screen = default_screen
        columns[SCREEN_NAME] = screen
        return columns