code: func (Actor) setBuildpack(config ApplicationConfig) types.FilteredString {
  	buildpacks := config.DesiredApplication.Buildpacks
  
  	if len(buildpacks) == 1 {
  		var filtered types.FilteredString
  		filtered.ParseValue(buildpacks[0])
  		return filtered
  	}
  
  	if buildpacks != nil && len(buildpacks) == 0 {
  		return types.FilteredString{IsSet: true}
  	}
  
  	return config.DesiredApplication.Buildpack
  }
docs: // If 'buildpacks' is set with only one buildpack, set `buildpack` (singular)
  // on the application.