funcsAO.go
func CharAt(s string, index int) string {
	l := len(s)
	shortcut := index < 0 || index > l-1 || l == 0
	if shortcut {
		return ""
	}
	return s[index : index+1]
}
func CharAt(s string, index int) string {
	l := len(s)
	shortcut := index < 0 || index > l-1 || l == 0
	if shortcut {
		return ""
	}
	return s[index : index+1]
}
// CharAt returns a string from the character at the specified position.