@staticmethod
def merge_signals(model_1, model_2):
    """
        Merge list of signals used.
        :param model_1: first model
        :param model_2: second model
        :return: union of signals
        """
    signals_1 = set(model_1.data['signals'])
    signals_2 = set(model_2.data['signals'])
    return list(signals_1.union(signals_2))
