anchor.diff:      /// No exception is thrown if the set already contains the value.
       let rec add value (tree : CharDiet) : CharDiet =
           // Preconditions
  -        //assert (dietInvariant tree)
  -        //assert (fst <| intervalsDisjoint tree Set.empty)
  +        assert (dietInvariant tree)
  +        //assert (intervalsDisjoint tree)
   
           match tree with
           | Empty ->
  
positive.code:         // Preconditions
          if a > b then
              invalidArg "b" "The upper bound of the interval is less than the lower bound."
          //assert (dietInvariant tree)
          //assert (fst <| intervalsDisjoint tree Set.empty)
  
          match tree with
          | Empty ->
  
negative.code:     let iter (action : char -> unit) (tree : CharDiet) =
          // Preconditions
          assert (dietInvariant tree)
          assert (intervalsDisjoint tree)
  
          /// Applies the action to all values within an interval.
          let intervalApplicator (lo, hi) =
  