func RandString(r *rand.Rand, n int) string {
	b := make([]byte, n)
	for i := range b {
		b[i] = letters[r.Intn(len(letters))]
	}
	return string(b)
}
func RandString(r *rand.Rand, n int) string {
	b := make([]byte, n)
	for i := range b {
		b[i] = letters[r.Intn(len(letters))]
	}
	return string(b)
}
// RandString returns a random alphabetical string of size n