input: def predict_temperature(temperature_list, alpha=0.5):
      """ Uses the exponential smoothing algorithm to predict the next day's forecasted temperature given the past three day's forecasted temperatures.
      >>> predict_temperature([21, 22, 20])
      21.0
      >>> predict_temperature([15, 17, 16], 0.3)
      15.9
      """
output:     if len(temperature_list) < 3:
          raise ValueError("The list must contain at least three temperatures.")
      
      smoothed_value = temperature_list[-1]
      for i in range(2, -1, -1):
          smoothed_value = alpha * temperature_list[i] + (1 - alpha) * smoothed_value
      
      return smoothed_value