private final CountDownLatch returned = new CountDownLatch(1);
    private Object proxy;

    @SuppressWarnings("unchecked") // proxy for a generic class
    <V> Callable<V> waitFor(Callable<V> callable) {
      return waitFor(callable, Callable.class);
    }

    @SuppressWarnings("unchecked") // proxy for a generic class
    <V> ClosingCallable<V> waitFor(ClosingCallable<V> closingCallable) {

    private final CountDownLatch returned = new CountDownLatch(1);
    private Object proxy;

    @SuppressWarnings("unchecked") // proxy for a generic class
    <V> Callable<V> waitFor(Callable<V> callable) {
      return waitFor(callable, Callable.class);
    }

    @SuppressWarnings("unchecked") // proxy for a generic class
    <V> ClosingCallable<V> waitFor(ClosingCallable<V> closingCallable) {

  }

  /**
   * The actual object that gets serialized. Unfortunately, readResolve() doesn't get called when a
   * circular dependency is present, so the proxy must be able to behave as the map itself.
   */
  abstract static class AbstractSerializationProxy<K, V> extends ForwardingConcurrentMap<K, V>
      implements Serializable {