docs: public Eval<V> nextOrNull(final K key) {
          final ValueSubscriber<V> sub = ValueSubscriber.subscriber();
          final LazyImmutable<Boolean> requested = LazyImmutable.def();
          return get(key).peek(a -> a.stream()
                                     .subscribe(sub))
                         .map(a -> Eval.always(() -> {
                             if (requested.isSet()) {
                                 sub.requestOne();
                             } else {
                                 requested.setOnce(true);
                             }
                             final Maybe<V> res = sub.toMaybe();
  
                             return res.orElse(null);
                         }))
  
                         .orElse(Eval.<V> now(null));
      }