procedure rrot( var t : tree );
     var temp : tree;
     begin
          temp := t;
          t := t^.left;
          temp^.left := t^.right;
          t^.right := temp;
          {*** adjust weight ***}
          t^.weight := temp^.weight;
          temp^.weight := wt(temp^.left) + wt(temp^.right);
     end;