anchor.diff:              if p < pred x then
                   let p', left' = find_del_right p left
                   assert (dietInvariant left')
  -                p', tap (fun x -> assert (dietInvariant x)) <| CharDiet.Join (comparer, left', right, (x, y))
  +                assert (fst <| intervalsDisjoint left' Set.empty)
  +
  +                p',
  +                CharDiet.Join (comparer, left', right, (x, y))
  +                |> tap (fun x ->
  +                    assert (dietInvariant x)
  +                    assert (fst <| intervalsDisjoint x Set.empty))
               elif p > y then
                   find_del_right p right
               else
  
positive.code:     let bounds (tree : CharDiet) : char * char =
          // Preconditions
          assert (dietInvariant tree)
  
          match tree with
          | Empty ->
  
negative.code:     let minElement (tree : CharDiet) : char =
          // Preconditions
          assert (dietInvariant tree)
          assert (fst <| intervalsDisjoint tree Set.empty)
  
          match tree with
          | Empty ->
  