code: public static <T> Reducer<PersistentBag<T>,T> toPersistentBag() {
          return Reducer.fromMonoid(Monoids.concatPersistentCollection(Bag.empty()), a -> Bag.of(a));
      }
docs: <pre>
  {@code
  PersistentBag<Integer> q = Reducers.toPersistentBag()
  .foldMap(Stream.of(1,2,3,4));
  
  }
  </pre>
  @return Reducer for PBag