code: public final <R> ParallelFlux<R> flatMap(Function<? super T, ? extends Publisher<? extends R>> mapper) {
  		return flatMap(mapper,
  				false,
  				Integer.MAX_VALUE, Queues.SMALL_BUFFER_SIZE);
  	}
docs: Generates and flattens Publishers on each 'rail'.
  <p>
  Errors are not delayed and uses unbounded concurrency along with default inner
  prefetch.
  
  @param <R> the result type
  @param mapper the function to map each rail's value into a Publisher
  
  @return the new {@link ParallelFlux} instance