leibnitz27

  {
    if (!(obj instanceof String s)) return;

    // s exists now.
  }

  public class InstanceOfPatternTest10 {
    static String s = "WIBBLE";

    public static void test(Object obj) {
        if (!(obj instanceof String s)) {
            throw new IllegalStateException();
        }
        System.out.println(s);
    }

    public static void test2(Object obj) {
        if (!(obj instanceof String s)) {
            if(true) {
                throw new IllegalStateException();
            }
        }
        System.out.println(s);
    }

    public static void main(String ... args) {
        test("Fred");
        test2("Fred");
    }
}

        static IEnumerable<int> infinite()
        {
            int x = 2;
            while (true)
            {
                yield return x++;
            }
        }

        static IEnumerable<int> primes(IEnumerable<int> l)
        { 
            int head = l.First();
            yield return head;
            foreach (var x in primes(l.Skip(1).Where(x => x % head != 0)))
                yield return x;
        }


        static void Main(string[] args)
        {
            System.Console.WriteLine(string.Join(",", primes(infinite()).Take(20).Select(x => x.ToString()))); 
        }