anchor.diff:      static member Iter (action, charSet : CharSet) : unit =
           // Preconditions
           checkNonNull "charSet" charSet
  +        assert (CharDiet.dietInvariant charSet.Tree)
   
           CharDiet.iter action charSet.Tree
   
  
positive.code:         | _, Empty ->
              Empty, head, stream
          | Some (x, y), Node (left, right, (a, b), _) ->
              let left', head, stream =
                  if x < a then
                      union' left (Some <| pred a) head stream
                  else
                      left, head, stream
              union_helper left' (a, b) right limit head stream
  
      /// Helper function for computing the union of two sets.
      and private union_helper left (a, b) (right : CharDiet) limit head stream =
  
negative.code:                 (Empty, sequence)
                  ||> Seq.fold (fun tree el ->
                      CharDiet.add el tree)
              assert (CharDiet.dietInvariant tree)
              CharSet (tree)
  
      //
  