@CheckReturnValue
    @BackpressureSupport(BackpressureKind.FULL)
    @SchedulerSupport(SchedulerSupport.NONE)
    public final <R> Flowable<R> concatMapMaybe(Function<? super T, ? extends MaybeSource<? extends R>> mapper) {
        return concatMapMaybe(mapper, 2);
    }