def _check_arg(arg):
    """Checks if the argument is True bool or string meaning True"""

    true_strings = ('True', 'true', 'T', 't', '.true.', '.True.')

    if arg is None:
        return 'n'
    else:
        if isinstance(arg, bool):
            if arg:
                return 'y'
            else:
                return 'n'
        if isinstance(arg, str):
            if arg in true_strings:
                return 'y'
            else:
                return 'n'
        return 'add'