anchor.diff:                  | Empty ->
                       left, None, Empty
                   | _ ->
  -                    AvlTree.TryExtractMin stream
  +                    AvlTree.TryDeleteMin stream
                       ||> inter_helper (a, b) right left
               elif b < x then
                   let right, head, stream = inter' right head stream
  
positive.code:                 | Empty ->
                      AvlTree.Create ((value, y), Empty, right)
                  | _ ->
                      let (u, v), l = AvlTree.ExtractMax left
                      if succ v = value then
                          AvlTree.Join (comparer, l, right, (u, y))
                      else
  
negative.code:         | Empty, Empty -> 0
          | Empty, Node (_,_,_,_) -> -1
          | Node (_,_,_,_), Node (_,_,_,_) ->
              let (ix1, iy1), r1 = AvlTree.DeleteMin t1
              let (ix2, iy2), r2 = AvlTree.DeleteMin t2
              let c =
                  match compare ix1 ix2 with
                  | 0 -> compare iy1 iy2
  