Get a list of non-existing string subtag(s) given the input string subtag(s).

        :param subtags: string subtag or a list of string subtags.
        :return: list of non-existing string subtags. The return list can be empty.

def filter(subtags):
        """
        Get a list of non-existing string subtag(s) given the input string subtag(s).

        :param subtags: string subtag or a list of string subtags.
        :return: list of non-existing string subtags. The return list can be empty.
        """
        if not isinstance(subtags, list):
            subtags = [subtags]
        return [subtag for subtag in subtags if len(tags.types(subtag)) == 0]