docs: public final static <T, R> Stream<R> flatMapStream(final Stream<T> stream, final Function<? super T, ? extends BaseStream<? extends R, ?>> fn) {
          return stream.flatMap(fn.andThen(bs -> {
  
              if (bs instanceof Stream)
                  return (Stream<R>) bs;
              else
                  return Streams.stream(bs.iterator());
  
          }));
      }