*     FutureCombiner}). Even if you don't care about the value of the future, you should
     *     typically check whether it failed: See <a
     *     href="https://errorprone.info/bugpattern/FutureReturnValueIgnored">https://errorprone.info/bugpattern/FutureReturnValueIgnored</a>.
     */
    public <C extends @Nullable Object> ListenableFuture<C> callAsync(
        AsyncCallable<C> combiner, Executor executor) {