anchor.diff:      let exists (predicate : char -> bool) (tree : CharDiet) =
           // Preconditions
           assert (dietInvariant tree)
  +        assert (intervalsDisjoint tree)
   
           // OPTIMIZE : Rewrite this to short-circuit and return early
           // if we find a non-matching element.
  
positive.code:             #endif
              result
  
      /// Helper function for computing the difference of two sets.
      let rec private diff' (input : CharDiet) head (stream : CharDiet) : CharDiet * (char * char) option * CharDiet =
          // Preconditions
          //assert (dietInvariant input)
          //assert (dietInvariant stream)
          //assert (fst <| intervalsDisjoint input Set.empty)
          //assert (fst <| intervalsDisjoint stream Set.empty)
  
          match head, input with
          | None, _->
  
negative.code:                 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
  