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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 top_level(self):
 """ Output the top level cells from the GDSII data. Top level cells are those that are not referenced by any other cells. Returns ------- out : list List of top level cells. """

        top = list(self.cell_dict.values())
        for cell in self.cell_dict.values():
            for dependency in cell.get_dependencies():
                if dependency in top:
                    top.remove(dependency)
        return top