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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 get_downbeat_steps(self):
 """ Return the indices of time steps that contain downbeats. Returns ------- downbeat_steps : list The indices of time steps that contain downbeats. """

        if self.downbeat is None:
            return []
        downbeat_steps = np.nonzero(self.downbeat)[0].tolist()
        return downbeat_steps