docs: def save_four_ds9(rectwv_coeff, debugplot=0):
      """Save the 4 possible ds9 region files.
  
      Parameters
      ----------
      rectwv_coeff : RectWaveCoeff instance
          Rectification and wavelength calibration coefficients for the
          particular CSU configuration.
      debugplot : int
              Debugging level for messages and plots. For details see
              'numina.array.display.pause_debugplot.py'.
  
      """
  
      for limits, rectified, suffix in zip(
          ['frontiers', 'frontiers', 'boundaries', 'boundaries'],
          [False, True, False, True],
          ['rawimage', 'rectified', 'rawimage', 'rectified']
      ):
          output = rectwv_coeff_to_ds9(rectwv_coeff=rectwv_coeff,
                                       limits=limits,
                                       rectified=rectified)
          filename = 'ds9_' + limits + '_' + suffix + '.reg'
          if abs(debugplot) >= 10:
              print('>>> Saving: ', filename)
          save_ds9(output, filename)