anchor.diff:              { new Collector<'T, ParStream<'Key * 'State>> with
                   member self.DegreeOfParallelism = stream.DegreeOfParallelism
                   member self.Iterator() = 
  -                    {   Index = ref -1; 
  -                        Func =
  +                    {   Func =
                               (fun value -> 
                                       let mutable grouping = Unchecked.defaultof<_>
                                       let key = projection value
  
positive.code:                         member self.DegreeOfParallelism = collector.DegreeOfParallelism
                          member self.Iterator() = 
                              let { Func = iter } as iterator = collector.Iterator()
                              {   Index = iterator.Index; 
                                  Func = (fun value -> iter (f value));
                                  Cts = iterator.Cts }
                          member self.Result = collector.Result }
                  stream.Apply collector }
  
negative.code: 
  /// Represents the iteration function
  type ParIterator<'T> = {
      /// The composed continutation with 'T for the current value
      Func : ('T -> unit)
      /// The current CancellationTokenSource
  