code: def slope_percentile(self, date, mag):
          
  
          date_diff = date[1:] - date[:len(date) - 1]
          mag_diff = mag[1:] - mag[:len(mag) - 1]
  
          # Remove zero mag_diff.
          index = np.where(mag_diff != 0.)
          date_diff = date_diff[index]
          mag_diff = mag_diff[index]
  
          # Derive slope.
          slope = date_diff / mag_diff
  
          percentile_10 = np.percentile(slope, 10.)
          percentile_90 = np.percentile(slope, 90.)
  
          return percentile_10, percentile_90
docs: Return 10% and 90% percentile of slope.
  
          Parameters
          ----------
          date : array_like
              An array of phase-folded date. Sorted.
          mag : array_like
              An array of phase-folded magnitudes. Sorted by date.
  
          Returns
          -------
          per_10 : float
              10% percentile values of slope.
          per_90 : float
              90% percentile values of slope.