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)