private void AddCategory(long?parent_id, XmlElement el)
        {
            string category_name = el.GetAttribute("name"); // string.Format("{0}.{1}", parent_name, el.GetAttribute("name"));
            string is_sealed_txt = el.GetAttribute("sealed");
            bool   is_sealed     = (is_sealed_txt != "");
            long   category_id   = dictionaryDBHelper_import.AddCategory(parent_id, category_name, is_sealed);

            foreach (XmlNode xml_category in el.SelectNodes("category"))
            {
                AddCategory(category_id, (XmlElement)xml_category);
            }

            /*
             *          foreach (XmlElement xml_category in el.GetElementsByTagName("category"))
             *          {
             *              AddCategory(category_id, xml_category);
             *          }*/

            foreach (XmlNode xml_question in el.SelectNodes("question"))
            {
                AddQuestion(category_id, (XmlElement)xml_question);
            }

            /*foreach (XmlElement xml_question in el.GetElementsByTagName("question"))
             * {
             *  AddQuestion(category_id, xml_question);
             * }*/
        }