reactor-core/src/main/java/reactor/core/publisher/ParallelFlux.java
public final <R> ParallelFlux<R> concatMap(Function<? super T, ? extends Publisher<? extends R>> mapper) {
		return concatMap(mapper, 2, ErrorMode.IMMEDIATE);
	}
public final <R> ParallelFlux<R> concatMap(Function<? super T, ? extends Publisher<? extends R>> mapper) {
		return concatMap(mapper, 2, ErrorMode.IMMEDIATE);
	}
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