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