def yes_no(prompt=''):
    """Prompts user with yes/no question.
    Args:
        prompt (str) opt: Question text. Defaults to ''.
    Returns:
        bool: Yes=True, No=False
    """
    while True:
        if prompt:
            print(prompt)
        answer = input('>>> ').lower()
        if answer in ['y', 'yes', 'ok', '']:
            return True
        if answer in ['n', 'no']:
            return False
        print('Please, answer \'yes\' or \'no\'.')