anchor.diff:          : CharDiet * (char * char) option * CharDiet =
           // Preconditions
           assert (AvlTree.Height input >= AvlTree.Height stream)
  -        assert (AvlTree.AvlInvariant input)
  -        assert (AvlTree.AvlInvariant stream)
  +        assert (dietInvariant input)
  +        assert (dietInvariant stream)
   
           match head, input with
           | None, _ ->
  
positive.code:     /// Helper function for computing the difference of two sets.
      and private diff_helper (a, b) (right : CharDiet) (left : CharDiet) head stream =
          // Preconditions
          assert (AvlTree.AvlInvariant left)
          assert (AvlTree.AvlInvariant right)
          assert (AvlTree.AvlInvariant stream)
  
          match head with
          | None ->
  
negative.code:     //
      let rec internal (*private*) find_del_right p (tree : CharDiet) : char * CharDiet =
          // Preconditions
          assert (dietInvariant tree)
  
          match tree with
          | Empty ->
  