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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 register_element(self, model, idx):
 """ Register element with index ``idx`` to ``model`` :param model: model name :param idx: element idx :return: final element idx """


        if idx is None:
            idx = model + '_' + str(len(self._idx_model))

        self._idx_model[idx] = model
        self._idx.append(idx)

        return idx