anchor.diff:          comparison t1 t2 = 0
   
       /// Returns the number of elements in the set.
  -    let count (t : CharDiet) =
  +    let count (tree : CharDiet) =
  +        // Preconditions
  +        assert (AvlTree.AvlInvariant tree)
  +
           // OPTIMIZE : Modify this to use a mutable stack instead of an F# list.
           let rec cardinal_aux acc = function
               | [] -> acc
  
positive.code:             elif y < b then
                  // [a, b] and [x, y] overlap
                  // y < b
                  let head, stream = AvlTree.TryExtractMin stream
                  diff_helper (succ y, b) right left head stream
              else
                  // [a, b] and [x, y] overlap
                  let right, head, stream = diff' right head stream
  
negative.code: 
      //
      let toList (tree : CharDiet) =
          // Preconditions
          assert (AvlTree.AvlInvariant tree)
  
          ([], tree)
          ||> fold (fun list el ->
              el :: list)
  
      //
      let toArray (tree : CharDiet) =
          // Preconditions
          assert (AvlTree.AvlInvariant tree)
  
          let elements = ResizeArray ()
          iter elements.Add tree
          elements.ToArray ()
  
      //
      let toSet (tree : CharDiet) =
          // Preconditions
          assert (AvlTree.AvlInvariant tree)
  
          (Set.empty, tree)
          ||> fold (fun set el ->
              Set.add el set)
  