code: def map_borders(wls):
      
      midpt_wl = 0.5 * (wls[1:] + wls[:-1])
      all_borders = np.zeros((wls.shape[0] + 1,))
      all_borders[1:-1] = midpt_wl
      all_borders[0] = 2 * wls[0] - midpt_wl[0]
      all_borders[-1] = 2 * wls[-1] - midpt_wl[-1]
      return all_borders
docs: Compute borders of pixels for interpolation.
  
      The border of the pixel is assumed to be midway of the wls