anchor.diff:  
   /// The renderer. Represents the rendering system in Nu generally.
   and Renderer =
  -    /// The render context for custom rendering.
  -    abstract RenderContext : obj
  -    /// Attempt to load a render asset.
  -    abstract TryLoadRenderAsset : obj AssetTag -> RenderAsset option
       /// Pop all of the physics messages that have been enqueued.
       abstract PopMessages : unit -> RenderMessage List
       /// Clear all of the render messages that have been enqueued.
  
positive.code: 
      interface Renderer with
  
          member renderer.RenderContext =
              renderer.RenderContext :> obj
  
          member renderer.TryLoadRenderAsset assetTag =
              SdlRenderer.tryLoadRenderAsset assetTag renderer
  
          member renderer.PopMessages () =
              let messages = renderer.RenderMessages
              renderer.RenderMessages <- List ()
  
negative.code:         { MockRenderer : unit }
  
      interface Renderer with
          member renderer.PopMessages () = List ()
          member renderer.ClearMessages () = ()
          member renderer.EnqueueMessage _ = ()
  