@SuppressWarnings("AnonymousToLambda") // We want an instance that can be GCed.
    AsyncCallable<Long> combiner =
        new AsyncCallable<Long>() {
          @Override
          public ListenableFuture<Long> call() {
            return SettableFuture.create();
          }
        };
    WeakReference<AsyncCallable<Long>> combinerRef = new WeakReference<>(combiner);

    ListenableFuture<Long> unused =

    @SuppressWarnings("AnonymousToLambda") // We want an instance that can be GCed.
    AsyncCallable<Long> combiner =
        new AsyncCallable<Long>() {
          @Override
          public ListenableFuture<Long> call() {
            return SettableFuture.create();
          }
        };
    WeakReference<AsyncCallable<Long>> combinerRef = new WeakReference<>(combiner);

    ListenableFuture<Long> unused =

     */
    public final <V extends @Nullable Object> ClosingFuture<V> callAsync(
        AsyncCombiningCallable<V> combiningCallable, Executor executor) {
      AsyncCallable<V> asyncCallable =
          new AsyncCallable<V>() {
            @Override
            public ListenableFuture<V> call() throws Exception {
              return new Peeker(inputs).callAsync(combiningCallable, closeables);
            }