code: public final <R> ParallelFlux<R> flatMap(Function<? super T, ? extends Publisher<? extends R>> mapper,
  			boolean delayError,
  			int maxConcurrency,
  			int prefetch) {
  		return onAssembly(new ParallelFlatMap<>(this,
  				mapper,
  				delayError,
  				maxConcurrency,
  				Queues.get(maxConcurrency),
  				prefetch, Queues.get(prefetch)));
  	}
docs: Generates and flattens Publishers on each 'rail', optionally delaying errors,
  having a total number of simultaneous subscriptions to the inner Publishers and
  using the given prefetch amount for the inner Publishers.
  
  @param <R> the result type
  @param mapper the function to map each rail's value into a Publisher
  @param delayError should the errors from the main and the inner sources delayed
  till everybody terminates?
  @param maxConcurrency the maximum number of simultaneous subscriptions to the
  generated inner Publishers
  @param prefetch the number of items to prefetch from each inner Publisher
  
  @return the new {@link ParallelFlux} instance