@NullUnmarked
public class SynchronizedDequeTest extends TestCase {

  protected Deque<String> create() {
    TestDeque<String> inner = new TestDeque<>();
    Deque<String> outer = Synchronized.deque(inner, inner.mutex);
    outer.add("foo"); // necessary because we try to remove elements later on
    return outer;
  }

  private static final class TestDeque<E> implements Deque<E> {
    private final Deque<E> delegate = new LinkedList<>();