How to slice a substring from Swift string

extension StringProtocol {
    func slice(_ start: Int, _ end: Int) -> SubSequence {
        let lower = index(self.startIndex, offsetBy: start)
        let upper = index(lower, offsetBy: end - start)
        return self[lower...upper]
    }
}

"BibinJ".slice(2, 4) // Answer is "bin"