anchor.diff:      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.
  
positive.code:         // Preconditions
          //assert (AvlTree.AvlInvariant l)
          //assert (AvlTree.AvlInvariant r)
          assert (AvlTree.Height l <= AvlTree.Height r + balanceTolerance)
  
          if AvlTree.Height l = AvlTree.Height r + balanceTolerance then
              match l with
  
negative.code:         // Preconditions
          //assert (AvlTree.AvlInvariant l)
          //assert (AvlTree.AvlInvariant r)
          //assert (AvlTree.Height r <= AvlTree.Height l + balanceTolerance)
  
          if AvlTree.Height r = AvlTree.Height l + balanceTolerance then
              match r with
  