anchor.diff:      /// Creates a AvlTree whose root node holds the specified value
       /// and the specified left and right subtrees.
       [<Pure>]
  -    static member inline Create (value, l, r : AvlTree<'T>) =
  +    static member inline (*private*) Create (l, r : AvlTree<'T>, value) =
           //assert (AvlTree.AvlInvariant l)
           //assert (AvlTree.AvlInvariant r)
  -        //assert (AvlTree.HeightDiff (l, r) <= balanceTolerance)
  +        assert (AvlTree.HeightDiff (l, r) <= balanceTolerance)
           Node (l, r, value, (max (AvlTree.Height l) (AvlTree.Height r)) + 1u)
   
       /// Creates a AvlTree containing the specified value.
       [<Pure>]
       static member Singleton value : AvlTree<'T> =
  -        AvlTree.Create (value, Empty, Empty)
  +        AvlTree.Create (Empty, Empty, value)
   
       //
       static member private mkt_bal_l (n, l, r : AvlTree<'T>) =
  
positive.code:             //&& (intervalsDisjoint tree Set.empty |> fst)
  
      //
      let rec internal (*private*) find_del_left p (tree : CharDiet) : char * CharDiet =
          // Preconditions
          assert (dietInvariant tree)
  
negative.code:                         failwith "Balance"
                      | Node (lrl, lrr, lrn, _) ->
                          AvlTree.Create (
                              AvlTree.Create (ll, lrl, ln),
                              AvlTree.Create (lrr, r, n),
                              lrn)
                      
          elif rh > lh + balanceTolerance then // right is heavier than left
              match r with
  