code: func (s SourceFilesystems) ContentStaticAssetFs(lang string) afero.Fs {
  	staticFs := s.StaticFs(lang)
  
  	base := afero.NewCopyOnWriteFs(s.Assets.Fs, staticFs)
  	return afero.NewCopyOnWriteFs(base, s.Content.Fs)
  
  }
docs: // ContentStaticAssetFs will create a new composite filesystem from the content,
  // static, and asset filesystems. The site language is needed to pick the correct static filesystem.
  // The order is content, static and then assets.
  // TODO(bep) check usage