anchor.diff:      let empty : AvlTree<'T> = AvlTree.Empty
   
       /// Returns the height of an AVL tree's root node.
  -    let rec private ht (tree : AvlTree<'T>) =
  +    let private ht (tree : AvlTree<'T>) =
           match tree with
           | Empty -> 0u
           | Node (_,_,_,h) -> h
  
positive.code:                     (AvlTree.Empty, head, stream)
                  | AvlTree.Node (rangeAB : Range<_>, left, right, _) ->
                      let left, head, stream =
                          if measurer.Compare (rangeXY.MinValue, rangeAB.MinValue) < 0 then
                              diff' left head stream
                          else
                              left, head, stream
  
negative.code: 
      /// Computes the union of the two sets.
      let rec union (measurer : IMeasurer<_>) (input : Diet<_>) (stream : Diet<_>) : Diet<'T> =
          let rec union' (input : Diet<_>) limit head (stream : Diet<_>) =
              match head with
              | None ->
  