code: public final <R> ParallelFlux<R> concatMap(Function<? super T, ? extends Publisher<? extends R>> mapper) {
  		return concatMap(mapper, 2, ErrorMode.IMMEDIATE);
  	}
docs: Generates and concatenates Publishers on each 'rail', signalling errors immediately
  and generating 2 publishers upfront.
  
  @param <R> the result type
  @param mapper the function to map each rail's value into a Publisher source and the
  inner Publishers (immediate, boundary, end)
  
  @return the new {@link ParallelFlux} instance