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:         | SE x -> rm x.DomElem
          | ST x -> rm x.DomText
          | SV x -> remove x.CurrentSkel
      let rec ins sk =
          match sk with
          | S0 -> ()
  
negative.code:                 // to later descend into it
                  sk.VarNeedsVisit <- true
                  ver))
      Tr (skel, ver) // Finally, return the Var node, backed by the inner / 
                     // outer update view.
  
  /// Main DOM manipulation routine used in Var nodes: remove old, insert cur.
  let patch (parent: El) pos old cur =
      // Removal function via reference equality.
      let rm (el: Node) =
          if Object.ReferenceEquals(el.ParentNode, parent) then
              parent.RemoveChild(el) |> ignore
  
      // Removes a node from the tree
      let rec remove sk =
          match sk with
          | S0 -> ()
  