* for APIs like {@link #whenAllComplete whenAllComplete(...)}{@code .}{@link
   * FutureCombiner#call(Callable, Executor) call(...)}, where it is easy to use a new input from
   * the {@code call} implementation but forget to add it to the arguments of {@code
   * whenAllComplete}.
   *

    AtomicReference<ClosingFuture.Peeker> capturedPeeker = new AtomicReference<>();
    ClosingFuture<TestCloseable> closingFuture =
        ClosingFuture.whenAllComplete(ImmutableList.of(input1, input2Failed))
            .call(
                new CombiningCallable<TestCloseable>() {
                  @Override