src/main/java/io/reactivex/parallel/ParallelFlowable.java
@CheckReturnValue
    @NonNull
    public final <R> ParallelFlowable<R> concatMap(
            @NonNull Function<? super T, ? extends Publisher<? extends R>> mapper) {
        return concatMap(mapper, 2);
    }
@CheckReturnValue
    @NonNull
    public final <R> ParallelFlowable<R> concatMap(
            @NonNull Function<? super T, ? extends Publisher<? extends R>> mapper) {
        return concatMap(mapper, 2);
    }
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 ParallelFlowable instance