docs: public static <K, V> Reducer<PersistentMap<K, V>,Tuple2<K,V>> toPersistentMap() {
          return Reducer.of(HashMap.empty(), (final PersistentMap<K, V> a) -> b -> a.putAll(b), (in) -> {
              Tuple2<K, V> w = in;
              return HashMap.of((K) w._1(), (V) w._2());
  
          }