anchor.diff:      /// Returns the number of elements in the set.
       let count (tree : CharDiet) =
           // Preconditions
  -        //assert (dietInvariant tree)
  -        //assert (fst <| intervalsDisjoint tree Set.empty)
  +        assert (dietInvariant tree)
  +        assert (intervalsDisjoint tree)
   
           // OPTIMIZE : Modify this to use a mutable stack instead of an F# list.
           let rec cardinal_aux acc = function
  
positive.code:     let toSet (tree : CharDiet) =
          // Preconditions
          assert (dietInvariant tree)
  
          (Set.empty, tree)
          ||> fold (fun set el ->
  
negative.code: 
              let result, _, _ =
                  CharDiet.TryDeleteMin stream
                  ||> diffImpl input
              
              Debug.Assert (
                  dietInvariant result,
                  "The DIET invariant does not hold for the result.")
              Debug.Assert (
                  intervalsDisjoint result,
                  "The intervals in the DIET are not disjoint.")
  
              #if DEBUG
  