anchor.diff:  ////////////////////////////////////////////////////////////////////////////////
   // Zipper classes
   
  -/// Zipper over type `'w` pointint at any type of hole.
  +/// Zipper over type `'w` pointing at unknown type of hole.
   type [<AbstractClass>] Zipper<'w> =
     abstract DownHeadAny: unit -> option<Zipper<'w>>
     abstract DownHeadThe: unit -> option<Zipper<'w, 'w>>
  
positive.code: val inline prevAny: Zipper<'x> -> option<Zipper<'x>>
  
  ////////////////////////////////////////////////////////////////////////////////
  // Homogenous interface
  
  val toZipperThe: 'x -> Zipper<'x, 'x>
  
  
negative.code: val fromZipper: Zipper<'x> -> 'x
  
  ////////////////////////////////////////////////////////////////////////////////
  // Heterogeneous interface
  
  val toZipperAny: 'x -> Zipper<'x>
  
  