anchor.diff:          //assert (AvlTree.AvlInvariant l)
           //assert (AvlTree.AvlInvariant r)
           assert (AvlTree.HeightDiff (l, r) <= (balanceTolerance + 1u))
  +        // TODO : Assert all values in 'l' are less than all values in 'r', and also less than 'n'.
  +        //        Assert 'n' is less than all values in 'r'.
   
           let lh = AvlTree.Height l
           let rh = AvlTree.Height r
  
positive.code:                 else
                      match right with
                      | Empty ->
                          CharDiet.Create (left, Empty, (x, value))
                      | _ ->
                          let (u, v), r = CharDiet.DeleteMin right
                          if safePred u = value then
                              CharDiet.Join (comparer, left, r, (x, v))
                          else
                              CharDiet.Create (left, right, (x, value))
  
              elif value < safePred x then
                  CharDiet.Join (comparer, add value left, right, (x, y))
              else
                  match left with
                  | Empty ->
                      CharDiet.Create (Empty, right, (value, y))
                  | _ ->
                      let (u, v), l = CharDiet.DeleteMax left
                      if safeSucc v = value then
                          CharDiet.Join (comparer, l, right, (u, y))
                      else
                          CharDiet.Create (left, right, (value, y))
  
      /// Returns a new set with the specified range of values added to the set.
      /// No exception is thrown if any of the values are already contained in the set.
  
negative.code:         // Preconditions
          //assert (AvlTree.AvlInvariant l)
          //assert (AvlTree.AvlInvariant r)
          // TODO : Assert all values in 'l' are less than all values in 'r'.
  
          match l, r with
          | Empty, Empty ->
  