import java.util.concurrent.CompletableFuture;
import java.util.concurrent.ConcurrentSkipListSet;

public class JI9043738 {

	public static void main(String[] args) {
		int i = 0; 
		while (true) { 
			i++; 
			ConcurrentSkipListSet<String> set = new ConcurrentSkipListSet<>(); 
			set.add("a"); 
			CompletableFuture<Void> future = CompletableFuture.runAsync( 
					() -> set.clear()); 
			set.add("b"); 
			set.add("c"); 
			future.join(); 
			if (set.isEmpty() && set.contains("c")) { 
				System.out.println( 
						"**** An empty set contains 'c'! (Iteration " + i + ")"); 
				break; 
			} 
		} 

	}

}