Callable<T> task,
      final BlockingQueue<Future<T>> queue) {
    final ListenableFuture<T> future = executorService.submit(task);
    future.addListener(
        new Runnable() {
          @Override
          public void run() {
            queue.add(future);
          }
        },
        directExecutor());
    return future;
  }

  /**

    // TODO: test whether null is supported (create a Feature)
    /**
     * The elements to be returned by future calls to {@code next()}, with the first at the top of
     * the stack.
     */
    final Stack<E> nextElements = new Stack<>();

    /**
     * The elements to be returned by future calls to {@code previous()}, with the first at the top
     * of the stack.
     */

    // TODO: test whether null is supported (create a Feature)
    /**
     * The elements to be returned by future calls to {@code next()}, with the first at the top of
     * the stack.
     */
    final Stack<E> nextElements = new Stack<>();

    /**
     * The elements to be returned by future calls to {@code previous()}, with the first at the top
     * of the stack.
     */

   * MD5 (128 hash bits) hash function and the given secret key.
   *
   * <p>If you are designing a new system that needs HMAC, prefer {@link #hmacSha256} or other
   * future-proof algorithms <a
   * href="https://datatracker.ietf.org/doc/html/rfc6151#section-2.3">over {@code hmacMd5}</a>.
   *
   * @param key the secret key

   * today. (We could probably then make {@code TypeVariableImpl} implement {@code AnnotatedElement}
   * so that we get partial compile-time checking.)
   *
   * <p>This workaround should be removed at a distant future time when we no longer support Java
   * versions earlier than 8.
   */
  @SuppressWarnings("removal") // b/318391980
  private static final class TypeVariableInvocationHandler implements InvocationHandler {

 *       OptionalLong} and {@code OptionalDouble}, the use of which is recommended; Guava does not
 *       have these.
 * </ul>
 *
 * <p><b>There are no plans to deprecate this class in the foreseeable future.</b> However, we do
 * gently recommend that you prefer the new, standard Java class whenever possible.
 *
 * <p>See the Guava User Guide article on <a

   * values are merged using the specified merging function. If the merging function returns {@code
   * null}, then the collector removes the value that has been computed for the key thus far (though
   * future occurrences of the key would reinsert it).
   *
   * <p>Entries will appear in the encounter order of the first occurrence of the key.
   *
   * @since 33.2.0 (available since 21.0 in guava-jre)
   */

   * (...))}, or for creating an empty list then calling {@link Collections#addAll}. This method is
   * not actually very useful and will likely be deprecated in the future.
   */
  @SafeVarargs
  @GwtCompatible(serializable = true)
  @SuppressWarnings("nullness") // TODO: b/316358623 - Remove after checker fix.

 *       OptionalLong} and {@code OptionalDouble}, the use of which is recommended; Guava does not
 *       have these.
 * </ul>
 *
 * <p><b>There are no plans to deprecate this class in the foreseeable future.</b> However, we do
 * gently recommend that you prefer the new, standard Java class whenever possible.
 *
 * <p>See the Guava User Guide article on <a

     * assume that this field is non-null.
     */
    @CheckForNull ObjectCountHashMap<E> contents;

    /**
     * If build() has been called on the current contents multiset, we need to copy it on any future
     * modifications, or we'll modify the already-built ImmutableMultiset.
     */
    boolean buildInvoked = false;
    /**