code: func filterUnmountedVolumes(mountedVolumes sets.String, expectedVolumes []string) []string {
  	unmountedVolumes := []string{}
  	for _, expectedVolume := range expectedVolumes {
  		if !mountedVolumes.Has(expectedVolume) {
  			unmountedVolumes = append(unmountedVolumes, expectedVolume)
  		}
  	}
  	return unmountedVolumes
  }
docs: // filterUnmountedVolumes adds each element of expectedVolumes that is not in
  // mountedVolumes to a list of unmountedVolumes and returns it.