public static <V extends @Nullable Object> ListenableFuture<List<@Nullable V>> successfulAsList(
      ListenableFuture<? extends V>... futures) {
    /*
     * Another way to express this signature would be to bound <V> by @NonNull and accept
     * LF<? extends @Nullable V>. That might be better: There's currently no difference between the
     * outputs users get when calling this with <Foo> and calling it with <@Nullable Foo>. The only