func Substr(s string, index int, n int) string {
	L := len(s)
	if index < 0 || index >= L || s == "" {
		return ""
	}
	end := index + n
	if end >= L {
		end = L
	}
	if end <= index {
		return ""
	}
	return s[index:end]
}
func Substr(s string, index int, n int) string {
	L := len(s)
	if index < 0 || index >= L || s == "" {
		return ""
	}
	end := index + n
	if end >= L {
		end = L
	}
	if end <= index {
		return ""
	}
	return s[index:end]
}
// Substr returns a substring of s starting at index of length n.