docs: def sse(mean, estimator):
      """
      Description:
          Calculates the Sum of Squared Errors (SSE) of
          an estimation on flat numpy ndarrays.
      Parameters:
          mean:      actual value (numpy ndarray)
          estimator: estimated value of the mean (numpy ndarray)
      """
      return np.sum((np.asarray(estimator) - np.asarray(mean)) ** 2, axis=0)