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]
}
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.
Substr returns a substring of s starting at index of length n .