reactor-core/src/main/java/reactor/core/publisher/ParallelFlux.java
public final <R> ParallelFlux<R> flatMap(Function<? super T, ? extends Publisher<? extends R>> mapper) {
		return flatMap(mapper,
				false,
				Integer.MAX_VALUE, Queues.SMALL_BUFFER_SIZE);
	}
public final <R> ParallelFlux<R> flatMap(Function<? super T, ? extends Publisher<? extends R>> mapper) {
		return flatMap(mapper,
				false,
				Integer.MAX_VALUE, Queues.SMALL_BUFFER_SIZE);
	}
Generates and flattens Publishers on each 'rail'.
<p>
Errors are not delayed and uses unbounded concurrency along with default inner
prefetch.

@param <R> the result type
@param mapper the function to map each rail's value into a Publisher

@return the new {@link ParallelFlux} instance