anchor.diff:              SDL.SDL_SetRenderDrawBlendMode (renderContext, SDL.SDL_BlendMode.SDL_BLENDMODE_ADD) |> ignore
               let viewAbsolute = (Math.getViewAbsoluteI eyeCenter eyeSize).InvertedView ()
               let viewRelative = (Math.getViewRelativeI eyeCenter eyeSize).InvertedView ()
  -            messages.Sort SdlRenderer.compareDescriptors
  +            //messages.Sort (RenderLayeredMessageComparer ())
  +            let messages = messages.AsParallel().OrderBy(id, RenderLayeredMessageComparer ())
               for message in messages do
                   SdlRenderer.renderDescriptor (&viewAbsolute, &viewRelative, eyeCenter, eyeSize, message.RenderDescriptor, renderer)
           | _ ->
  
positive.code:     /// Handle render clean up by freeing all loaded render assets.
      abstract CleanUp : unit -> Renderer
  
  /// The mock implementation of Renderer.
  type [<ReferenceEquality; NoComparison>] MockRenderer =
      private
  
negative.code: namespace Nu
  open System
  open System.Collections.Generic
  open System.Linq
  open System.Numerics
  open System.IO
  open SDL2
  