public void addAll(IntTreeBag addend){
     IntBTNode addroot;
     
     if(addend == null){
         throw new IllegalArgumentException(" bag is empty");
     }
     if(root == addend.root){
         addroot = IntBTNode.treeCopy(addend.root);
         addTree(addroot);
     }else{
         addTree(addend.root);
     }
 }