static func getPrimes(to n: Int) -> [Int] {
    let xmody = (1...n)
        .map { x in (1...n).map { y in x % y } }

    let primes = xmody
        .map { mods in
            mods.enumerate()
            .filter { y, mod in mod == 0 }
            .map { y, mod in y + 1 } // divisors for x
        }
        .enumerate()
        .filter { x, zs in
            guard let z0 = zs.first, z1 = zs.last where zs.count <= 2 else {
                return false
            }
            return z0 == 1 && z1 == x + 1
        }
        .map { x, _ in x + 1 }

    return primes
}