code: public static final IntStream reducingStream(IntStream stream, ReducingFunction func)
      {
          return StreamSupport.intStream(new ReducingSpliterator(stream, func), false);
      }
docs: Converts Stream to Stream where one or more int items are mapped to
  one int item. Mapping is done by func method. Func must return
  true after calling consumer method. Otherwise behavior is unpredictable.
  
  @param stream
  @param func
  @return
  @throws IllegalStateException if Stream ends when mapping is unfinished.
  @deprecated This is not tested at all!!!