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:                     // balance left: combination
                      match rl with
                      | Empty ->
                          failwith "rebalance"
                      | Node (rll, rlr, rln, _) ->
                          AvlTree.Create (
                              rln,
  
negative.code:                     // balance right: combination
                      match lr with
                      | Empty ->
                          failwith "Balance"
                      | Node (lrl, lrr, lrn, _) ->
                          AvlTree.Create (
                              lrn,
  