code: 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);
  	}
docs: 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