def _fit_single_layer_loop(param):
    """
    Loop body to be passed to the parallel pool.

    Note: `func_fit_single_layer` can be:
        (1) helper_hh_model.fit_HH_x_single_layer(), or
        (2) helper_mkz_model.fit_H4_x_single_layer()
        etc.
    """
    damping_curve, other_params = param

    (
        func_fit_single_layer, use_scipy, pop_size, n_gen, lower_bound_power,
        upper_bound_power, eta, seed, show_fig, verbose,
    ) = other_params

    best_para = func_fit_single_layer(
        damping_curve,
        use_scipy=use_scipy,
        n_gen=n_gen,
        eta=eta,
        pop_size=pop_size,
        lower_bound_power=lower_bound_power,
        upper_bound_power=upper_bound_power,
        seed=seed,
        show_fig=show_fig,
        verbose=verbose,
        parallel=False,  # no par. within layers
    )

    return best_para