src/main/java/io/reactivex/parallel/ParallelFlowable.java
@CheckReturnValue
    @NonNull
    public final <R> ParallelFlowable<R> concatMapDelayError(
            @NonNull Function<? super T, ? extends Publisher<? extends R>> mapper,
                    boolean tillTheEnd) {
        return concatMapDelayError(mapper, 2, tillTheEnd);
    }
@CheckReturnValue
    @NonNull
    public final <R> ParallelFlowable<R> concatMapDelayError(
            @NonNull Function<? super T, ? extends Publisher<? extends R>> mapper,
                    boolean tillTheEnd) {
        return concatMapDelayError(mapper, 2, tillTheEnd);
    }
Generates and concatenates Publishers on each 'rail', optionally delaying errors
and generating 2 publishers upfront.

@param <R> the result type
@param mapper the function to map each rail's value into a Publisher
@param tillTheEnd if true all errors from the upstream and inner Publishers are delayed
till all of them terminate, if false, the error is emitted when an inner Publisher terminates.
source and the inner Publishers (immediate, boundary, end)
@return the new ParallelFlowable instance