anchor.diff:          if lh > rh + balanceTolerance then // left is heavier than right
               match l with
               | Empty ->
  -                failwith "rebalance"
  +                failwith "Balance"
               | Node (ll, lr, ln, _) ->
                   // one of the nodes must have height > height r + 1
                   if AvlTree.Height ll >= AvlTree.Height lr then
  
positive.code:         elif rh > lh + balanceTolerance then // right is heavier than left
              match r with
              | Empty ->
                  failwith "rebalance"
              | Node (rl, rr, rn, _) ->
                  // one of the nodes must have height > height t1 + 1
                  if AvlTree.Height rr >= AvlTree.Height rl then
  
negative.code:                     // balance left: combination
                      match rl with
                      | Empty ->
                          failwith "Balance"
                      | Node (rll, rlr, rln, _) ->
                          AvlTree.Create (
                              rln,
  