Public Extension Submission

Contents.swift

extension String {
    subscript(i: Int) -> Character {
        return self[index(startIndex, offsetBy: i)]
    }
    
    subscript(r: Range<Int>) -> String {
        return substring(with: index(startIndex, offsetBy: r.startIndex)..<index(startIndex, offsetBy: r.endIndex))
    }
}

extension Int {
    func isPrime() -> Bool {
        return self > 1 && !(2..<self).contains { self % $0 == 0 }
    }
}