code: def _shift_wavelengths(model1, model2):
      
      if isinstance(model1, _models.RedshiftScaleFactor):
          val = _get_sampleset(model2)
          if val is None:
              w = val
          else:
              w = model1.inverse(val)
      elif isinstance(model1, _models.Scale):
          w = _get_sampleset(model2)
      else:
          w = _get_sampleset(model1)
      return w
docs: One of the models is either ``RedshiftScaleFactor`` or ``Scale``.
  
      Possible combos::
  
          RedshiftScaleFactor | Model
          Scale | Model
          Model | Scale