public FlashcardList(XElement root)
        {
            _categories = new FlashcardCategoryTree(root.Element("categories"));

            var cardRoot = root.Element("cards");
            if (cardRoot == null)
                throw new ArgumentException(Resources.FlashcardList_FlashcardList_Missing_cards_from_Flashcard_XML_file, "root");

            _creator = root.Attribute("creator").Value;
        }
        public FlashcardList(XElement root)
        {
            _categories = new FlashcardCategoryTree(root.Element("categories"));

            var cardRoot = root.Element("cards");

            if (cardRoot == null)
            {
                throw new ArgumentException(Resources.FlashcardList_FlashcardList_Missing_cards_from_Flashcard_XML_file, "root");
            }

            _creator = root.Attribute("creator").Value;
        }