reactor-core/src/main/java/reactor/core/publisher/ParallelFlux.java
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)));
	}
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)));
	}
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