code: 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));
  	}
docs: 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