anchor.diff:    override o.OnRenderFrame(e) =
   
       GL.Clear(ClearBufferMask.ColorBufferBit ||| ClearBufferMask.DepthBufferBit)
  +    GL.ClearColor(0.1f, 0.2f, 0.5f, 0.0f)
   
       if continuous then
         try
  
positive.code:     air_bounds.prepare ()
  
      // set other GL states.
      GL.Enable(EnableCap.DepthTest)
      GL.BlendFunc(BlendingFactorSrc.SrcAlpha, BlendingFactorDest.One)
      GL.Enable(EnableCap.Blend)
      GL.ClearColor(0.1f, 0.2f, 0.5f, 0.0f)
      GL.LineWidth(2.0f);
  
      base.OnLoad e
  
negative.code:     shader_manager.set_projection base.Width base.Height
      shader_manager.set_model_view <| camera.matrix ()
      for location in solids do
        solid_bounds.render location false
      for location in fluid do
        fluid_bounds.render location false
      for location in air do
        air_bounds.render location false
      world_bounds.render ()
  
      // second pass (only if debugging).
      if debug_mode then
        GL.Disable(EnableCap.DepthTest)
        shader_manager.use_debug ()
        shader_manager.set_projection base.Width base.Height
        shader_manager.set_model_view <| camera.matrix ()
        for location in solids do
          solid_bounds.render location true
        for location in fluid do
          fluid_bounds.render location true
        for location in air do
          air_bounds.render location true
        GL.Enable(EnableCap.DepthTest)
  
      let code = GL.GetError ()
      if code <> ErrorCode.NoError then
  