ImmutableList<? extends ListenableFuture<?>> inputs =
        ImmutableList.<ListenableFuture<?>>of(future1, future2, future3);
    ImmutableList<ListenableFuture<Object>> futures = inCompletionOrder(inputs);
    future2.set("1L");
    future1.set(2L);
    future3.set(3);

    ImmutableList<?> expected = ImmutableList.of("1L", 2L, 3);

    ImmutableList<? extends ListenableFuture<?>> inputs =
        ImmutableList.<ListenableFuture<?>>of(future1, future2, future3);
    ImmutableList<ListenableFuture<Object>> futures = inCompletionOrder(inputs);
    future2.set("1L");
    future1.set(2L);
    future3.set(3);

    ImmutableList<?> expected = ImmutableList.of("1L", 2L, 3);

   * <p>Only for use by a {@link CombiningCallable} or {@link AsyncCombiningCallable} object.
   */
  public static final class Peeker {
    private final ImmutableList<ClosingFuture<?>> futures;
    private volatile boolean beingCalled;

    private Peeker(ImmutableList<ClosingFuture<?>> futures) {
      this.futures = checkNotNull(futures);
    }

    /**
     * Returns the value of {@code closingFuture}.
     *

   * <p>Only for use by a {@link CombiningCallable} or {@link AsyncCombiningCallable} object.
   */
  public static final class Peeker {
    private final ImmutableList<ClosingFuture<?>> futures;
    private volatile boolean beingCalled;

    private Peeker(ImmutableList<ClosingFuture<?>> futures) {
      this.futures = checkNotNull(futures);
    }

    /**
     * Returns the value of {@code closingFuture}.
     *