anchor.diff:      let rec internal avl = function
           | Empty -> true
           | Node (x, l, r, h) ->
  -            let height_l = height l
  -            let height_r = height r
  +            let height_l = computeHeight l
  +            let height_r = computeHeight r
               height_l = height_r
               || (height_l = (1u + height_r) || height_r = (1u + height_l))
               && h = ((max height_l height_r) + 1u)
  
positive.code:     let empty : AvlTree<'T> = AvlTree.Empty
  
      /// Returns the height of an AVL tree's root node.
      let private ht (tree : AvlTree<'T>) =
          match tree with
          | Empty -> 0u
          | Node (_,_,_,h) -> h
  
negative.code:         | Node (_,_,_,_) -> false
  
      /// Implementation. Returns the height of an AVL tree.
      let rec private computeHeightRec (tree : AvlTree<'T>) cont =
          match tree with
          | Empty ->
              cont 0u
          | Node (_, l, r, _) ->
              computeHeightRec l <| fun height_l ->
              computeHeightRec r <| fun height_r ->
                  (max height_l height_r) + 1u
                  |> cont
  
      /// Returns the height of an AVL tree.
      let internal computeHeight (tree : AvlTree<'T>) =
          computeHeightRec tree id
  
      /// Determines if an AVL tree is correctly formed.
      /// It isn't necessary to call this at run-time, though it may be useful for asserting
  