def __init__(self, groups=None):
        super(GroupedRequirements, self).__init__(NamedRequirements)
        self.groups = groups or self.default_groups
        self.filename = None
        self.create_default_groups()
def __notify(self):
        if self.__observer is not None:
            self.__observer.notify(self.__means, self.__variances, self.__clusters)