code: def spline_backwards_hankel(ht, htarg, opt):
      r
      # Ensure ht is all lowercase
      ht = ht.lower()
  
      # Only relevant for 'fht' and 'hqwe', not for 'quad'
      if ht in ['fht', 'qwe', 'hqwe']:
  
          # Get corresponding htarg
          if ht == 'fht':
              htarg = _check_targ(htarg, ['fhtfilt', 'pts_per_dec'])
          elif ht in ['qwe', 'hqwe']:
              htarg = _check_targ(htarg, ['rtol', 'atol', 'nquad', 'maxint',
                                  'pts_per_dec', 'diff_quad', 'a', 'b', 'limit'])
  
          # If spline (qwe, fht) or lagged (fht)
          if opt == 'spline':
  
              # Issue warning
              mesg = ("\n    The use of `opt='spline'` is deprecated and will " +
                      "be removed\n    in v2.0.0; use the corresponding " +
                      "setting in `htarg`.")
              warnings.warn(mesg, DeprecationWarning)
  
              # Reset opt
              opt = None
  
              # Check pts_per_dec; set to old default values if not given
              if 'pts_per_dec' not in htarg:
                  if ht == 'fht':
                      htarg['pts_per_dec'] = -1  # Lagged Convolution DLF
  
                  elif ht in ['qwe', 'hqwe']:
                      htarg['pts_per_dec'] = 80  # Splined QWE; old default value
  
      return htarg, opt
docs: r"""Check opt if deprecated 'spline' is used.
  
      Returns corrected htarg, opt.
      r