def process_keff_stddev(token_list, values):
    """Acquire standard deviation of keff result.

    This function parses a line, retrieves estimated standard deviation
    and stores it in the main 'values' dictionary with a key 'std_dev'.

    Arguments:
        token_list (list)[-]: A list of tokens to be processed.
        values (dict)[-]: Main dictionary to store processed values.

    Returns:
        values (dict)[-]: Main dictionary to store processed values.
        token_list (list)[-]: A list of processed tokens.

    """
    (keff, std_dev) = (float(token_list[2]), float(token_list[3]))

    values.setdefault('std_dev', []).append((keff, std_dev))
    return values, token_list