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