docs: def find_pix_borders(sp, sought_value):
      """Find useful region of a given spectrum
  
      Detemine the useful region of a given spectrum by skipping
      the initial (final) pixels with values equal to 'sought_value'.
  
      Parameters
      ----------
      sp : 1D numpy array
          Input spectrum.
      sought_value : int, float, bool
          Pixel value that indicate missing data in the spectrum.
  
      Returns
      -------
      jmin, jmax : tuple (integers)
          Valid spectrum region (in array coordinates, from 0 to
          NAXIS1 - 1). If the values of all the pixels in the spectrum 
          are equal to 'sought_value', the returned values are jmin=-1 
          and jmax=naxis1.
  
      """
  
      if sp.ndim != 1:
          raise ValueError('Unexpected number of dimensions:', sp.ndim)
      naxis1 = len(sp)
  
      jborder_min = -1
      jborder_max = naxis1
  
      # only spectra with values different from 'sought_value'
      if not np.alltrue(sp == sought_value):
          # left border
          while True:
              jborder_min += 1
              if sp[jborder_min] != sought_value:
                  break
          # right border
          while True:
              jborder_max -= 1
              if sp[jborder_max] != sought_value:
                  break
  
      return jborder_min, jborder_max