src/main/java/io/reactivex/parallel/ParallelFlowable.java
@CheckReturnValue
    @NonNull
    public final <R> ParallelFlowable<R> flatMap(
            @NonNull Function<? super T, ? extends Publisher<? extends R>> mapper, boolean delayError) {
        return flatMap(mapper, delayError, Integer.MAX_VALUE, Flowable.bufferSize());
    }
@CheckReturnValue
    @NonNull
    public final <R> ParallelFlowable<R> flatMap(
            @NonNull Function<? super T, ? extends Publisher<? extends R>> mapper, boolean delayError) {
        return flatMap(mapper, delayError, Integer.MAX_VALUE, Flowable.bufferSize());
    }
Generates and flattens Publishers on each 'rail', optionally delaying errors.
<p>
It 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
@param delayError should the errors from the main and the inner sources delayed till everybody terminates?
@return the new ParallelFlowable instance