reactor-core/src/main/java/reactor/core/publisher/ParallelFlux.java
final <R> ParallelFlux<R> concatMapDelayError(Function<? super T, ? extends
			Publisher<? extends R>> mapper,
			boolean delayUntilEnd,
			int prefetch) {
		return concatMap(mapper, prefetch, delayUntilEnd ? ErrorMode.END: ErrorMode.BOUNDARY);
	}
final <R> ParallelFlux<R> concatMapDelayError(Function<? super T, ? extends
			Publisher<? extends R>> mapper,
			boolean delayUntilEnd,
			int prefetch) {
		return concatMap(mapper, prefetch, delayUntilEnd ? ErrorMode.END: ErrorMode.BOUNDARY);
	}
Generates and concatenates Publishers on each 'rail', 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 delayUntilEnd true if delayed until all sources are concatenated
@param prefetch the number of items to prefetch from each inner Publisher
source and the inner Publishers (immediate, boundary, end)

@return the new {@link ParallelFlux} instance