def confirm(s: str = ''):
    """
    Ask yes/no, retry if invalid.

    :param str s: prompt
    :return: bool
    """
    while True:
        value = input('> {} [y/n]: '.format(s)).lower()
        if value:
            if value in 'yesrtui':
                return True
            elif value in 'novbm,':
                return False