code: func getSuffix(c context.Context) string {
  	const allowed = "abcdefghijklmnopqrstuvwxyz0123456789"
  	suf := make([]byte, 4)
  	for i := range suf {
  		suf[i] = allowed[mathrand.Intn(c, len(allowed))]
  	}
  	return string(suf)
  }
docs: // getSuffix returns a random suffix to use when naming a GCE instance.