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)
    return best_para
