/// given element.

     /// given element.

     int insert(const Item& item) {

     int insert(const Item& item) {

       int cdx = newClass();

       int cdx = newClass();

       classes[cdx].prev = -1;

       classes[cdx].prev = -1;

       classes[cdx].next = firstClass;

       classes[cdx].next = firstClass;

       firstClass = cdx;

       firstClass = cdx;

       int idx = newItem();

       int idx = newItem();

       items[idx].item = item;

       items[idx].item = item;

       items[idx].cls = cdx;

       items[idx].cls = cdx;