Return a newly created |data.Category| object having *label* and
        appended to the end of the sub-category sequence for this category.

def add_sub_category(self, label):
        """
        Return a newly created |data.Category| object having *label* and
        appended to the end of the sub-category sequence for this category.
        """
        category = Category(label, self)
        self._sub_categories.append(category)
        return category