anchor.diff:          lock dependencies (fun _ -> 
               let dep = dependencies.GetValue(source, createValueCallbackFor source) :?> DependencyTracker<'a>
               dep.Add(target, source))
  +
       static member internal Subscribe (source : ISignal<'a>, target : IObserver<'a>) =
           lock dependencies (fun _ -> 
               let dep = dependencies.GetValue(source, createValueCallbackFor source) :?> DependencyTracker<'a>
  
positive.code: 
  /// Type used to track dependencies
  type [<AllowNullLiteral>] IDependencyManager<'a> =
      /// Add a dependent to the source signal explicitly
      abstract member Add : IDependent * ISignal<'a> -> unit
  
  
negative.code: 
  /// Helper module for dealing with weak references
  module WeakRef =
  
      /// Convert the WeakReference to an option
      let toOption (wr : WeakReference<_>) =
          match wr.TryGetTarget() with
  