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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 _smooth_best_span_estimates(self):
 """Apply a MID_SPAN smooth to the best span estimates at each observation."""

        self._smoothed_best_spans = smoother.perform_smooth(self.x,
                                                            self._best_span_at_each_point,
                                                            MID_SPAN)