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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 display(self, depth=0):
 """ Pretty-prints this cluster. Useful for debuging. """

        print(depth * "    " + "[level %s]" % self.level)
        for item in self.items:
            if isinstance(item, Cluster):
                item.display(depth + 1)
            else:
                print(depth * "    " + "%s" % item)