anchor.diff:      //
       let rec private find_del_right p (tree : CharDiet) =
           match tree with
  -        | AvlTree.Empty ->
  -            p, AvlTree.Empty
  -        | AvlTree.Node ((x, y), left, right, _) ->
  +        | Empty ->
  +            p, Empty
  +        | Node ((x, y), left, right, _) ->
               if p < pred x then
                   let p', left' = find_del_right p left
                   p', AvlDiet.join (x, y) left' right
  
positive.code:         and union_helper left (a, b) right limit head stream =
              match head with
              | None ->
                  AvlDiet.join (a, b) left right, None, AvlTree.Empty
              | Some (x, y) ->
                  let greater_limit z =
                      match limit with
  
negative.code:     open OptimizedClosures
  
      /// An empty AVL tree.
      let empty : AvlTree<'T> = Empty
  
      /// Returns the height of an AVL tree's root node.
      let inline height (tree : AvlTree<'T>) =
  