reactor-core/src/main/java/reactor/core/publisher/ParallelFlux.java
final <R> ParallelFlux<R> concatMap(Function<? super T, ? extends Publisher<? extends R>> mapper,
			int prefetch,
			ErrorMode errorMode) {
		return onAssembly(new ParallelConcatMap<>(this,
				mapper,
				Queues.get(prefetch),
				prefetch,
				errorMode));
	}
final <R> ParallelFlux<R> concatMap(Function<? super T, ? extends Publisher<? extends R>> mapper,
			int prefetch,
			ErrorMode errorMode) {
		return onAssembly(new ParallelConcatMap<>(this,
				mapper,
				Queues.get(prefetch),
				prefetch,
				errorMode));
	}
Generates and concatenates Publishers on each 'rail', optionally delaying errors
and using the given prefetch amount for generating Publishers upfront.

@param <R> the result type
@param mapper the function to map each rail's value into a Publisher
@param prefetch the number of items to prefetch from each inner Publisher
@param errorMode the error handling, i.e., when to report errors from the main
source and the inner Publishers (immediate, boundary, end)

@return the new {@link ParallelFlux} instance