anchor.diff:        return Cons(hd, merge tl other) }
   
     /// Merges all specified async sequences into an async sequence non-deterministically.
  -  let rec mergeAll (ss:AsyncSeq<'a> list) : AsyncSeq<'a> =
  +  let rec mergeAll (ss:AsyncSeq<'T> list) : AsyncSeq<'T> =
       match ss with
       | [] -> empty
       | [s] -> s
  
positive.code:     module internal Choice =
    
        /// Maps over the left result type.
        let mapl (f:'a -> 'b) = function
          | Choice1Of2 a -> f a |> Choice1Of2
          | Choice2Of2 e -> Choice2Of2 e
  
        /// Maps over the right result type.
        let mapr (f:'b -> 'c) = function
          | Choice1Of2 a -> Choice1Of2 a
          | Choice2Of2 e -> f e |> Choice2Of2
  
      // ----------------------------------------------------------------------------
  
  
negative.code: 
        /// Creates an async computations which runs the specified computations
        /// in parallel and returns their results.
        static member Parallel(a:Async<'T>, b:Async<'U>) : Async<'T * 'U> = async {
          let! a = a |> Async.StartChild
          let! b = b |> Async.StartChild
          let! a = a
  