anchor.diff:  
   [<AutoOpen>]
   module internal Util =
  -  type IDictionary<'k, 'v> with
  -    member this.TryGet k =
  -      match this.TryGetValue k with
  -       | (false, _) -> None
  -       | (_, v) -> Some v
  -    member this.Add (kvs: seq<'k * 'v>) =
  -      kvs |> Seq.iter this.Add
  -  module Dictionary =
  -    let create kvs =
  -      let d = Dictionary<_, _>()
  -      d.Add kvs
  -      d
  -
     let setupWhen (onEvent: IEvent<_, _>) =
       let mainCtx = ivar ()
       let handler =
  
positive.code: // choice stream usage patterns.  WPF is fundamentally an imperative UI
  // framework and this example does not try to rewrite WPF as a more declarative
  // framework.  Some of the simple techniques used in this example, such using an
  // immutable list as the model and removing and adding all controls to a panel
  // when the model changes, do not scale (yes, there are builtin components for
  // this in WPF).  Consider those as exercises to fix.  Would have used XAML type
  // provider, but it currently does not mix with PCL.
  //
  // See also: https://github.com/tastejs/todomvc/blob/master/app-spec.md
  
negative.code:     Async.setMain ctx
      onEvent.RemoveHandler handler
  
    module Stream =
      module Var =
        let updateFun v f =
          Stream.Var.get v |> f |> Stream.Var.set v
  
  type Item = {
      IsCompleted: bool
      Header: string
    }
  
  module Id =
    let mutable private id = 0L
    let next () = Interlocked.Increment &id
  
  [<STAThread; EntryPoint>]
  let main argv =
  