code: def add_fixed_lens(self, kwargs_fixed_lens, kwargs_lens_init):
          
          kwargs_fixed_lens = self._solver.add_fixed_lens(kwargs_fixed_lens, kwargs_lens_init)
          return kwargs_fixed_lens
docs: returns kwargs that are kept fixed during run, depending on options
          :param kwargs_options:
          :param kwargs_lens:
          :return: