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.
  
positive.code:         { MockRenderer : unit }
  
      interface Renderer with
          member renderer.TryLoadRenderAsset _ = None
          member renderer.PopMessages () = List ()
          member renderer.ClearMessages () = ()
  
negative.code: 
      interface Renderer with
  
          member renderer.RenderContext =
              renderer.RenderContext :> obj
  
          member renderer.TryLoadRenderAsset assetTag =
              SdlRenderer.tryLoadRenderAsset assetTag renderer
  
  