code: public final <R> ParallelFlux<R> flatMap(Function<? super T, ? extends Publisher<? extends R>> mapper,
  			boolean delayError,
  			int maxConcurrency) {
  		return flatMap(mapper,
  				delayError,
  				maxConcurrency, Queues.SMALL_BUFFER_SIZE);
  	}
docs: Generates and flattens Publishers on each 'rail', optionally delaying errors and
  having a total number of simultaneous subscriptions to the inner Publishers.
  <p>
  It uses a default inner prefetch.
  
  @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
  
  @return the new {@link ParallelFlux} instance