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 start_date(self):
        """ Returns a date object of the todo's start date. """
        return self.get_date(config().tag_start())