anchor.diff:  
   /// Descends into Skel and updates dirty nodes.
   let update parent skel =
  +    // Gets the position of sk relative to a DOM node
       let rec posOf sk =
           match sk with
           | S0 -> AtEnd
  
positive.code:                 // to later descend into it
                  sk.VarNeedsVisit <- true
                  ver))
      Tr (skel, ver)
  
  /// Main DOM manipulation routine used in Var nodes: remove old, insert cur.
  let patch (parent: El) pos old cur =
      let rm (el: Node) =
          if Object.ReferenceEquals(el.ParentNode, parent) then
              parent.RemoveChild(el) |> ignore
      let rec remove sk =
          match sk with
          | S0 -> ()
  
negative.code:             VarNeedsVisit = true
          }
      let skel = SV sk
      // v is a reactive view of a Tree.
      // Whenever there's an outer change, we want to mark this node as dirty.
      // When there's an inner change, we mark that the children are dirty and
      // should be visited.
      let ver =
          v
          |> R.View.Bind (fun (Tr (ske, y)) ->
  