src/main/java/io/reactivex/Observable.java
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final <R> Observable<R> concatMapSingleDelayError(Function<? super T, ? extends SingleSource<? extends R>> mapper, boolean tillTheEnd, int prefetch) {
        ObjectHelper.requireNonNull(mapper, "mapper is null");
        ObjectHelper.verifyPositive(prefetch, "prefetch");
        return RxJavaPlugins.onAssembly(new ObservableConcatMapSingle<T, R>(this, mapper, tillTheEnd ? ErrorMode.END : ErrorMode.BOUNDARY, prefetch));
    }
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final <R> Observable<R> concatMapSingleDelayError(Function<? super T, ? extends SingleSource<? extends R>> mapper, boolean tillTheEnd, int prefetch) {
        ObjectHelper.requireNonNull(mapper, "mapper is null");
        ObjectHelper.verifyPositive(prefetch, "prefetch");
        return RxJavaPlugins.onAssembly(new ObservableConcatMapSingle<T, R>(this, mapper, tillTheEnd ? ErrorMode.END : ErrorMode.BOUNDARY, prefetch));
    }
Maps the upstream items into {@link SingleSource}s and subscribes to them one after the
other succeeds or fails, emits their success values and optionally delays  errors
till both this {@code Observable} and all inner {@code SingleSource}s terminate.
<p>
<img width="640" height="305" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/concatMap.png" alt="">
<dl>
<dt><b>Scheduler:</b></dt>
<dd>{@code concatMapSingleDelayError} does not operate by default on a particular {@link Scheduler}.</dd>
</dl>
<p>History: 2.1.11 - experimental
@param <R> the result type of the inner {@code SingleSource}s
@param mapper the function called with the upstream item and should return
a {@code SingleSource} to become the next source to
be subscribed to
@param tillTheEnd If {@code true}, errors from this {@code Observable} or any of the
inner {@code SingleSource}s are delayed until all
of them terminate. If {@code false}, an error from this
{@code Observable} is delayed until the current inner
{@code SingleSource} terminates and only then is
it emitted to the downstream.
@param prefetch The number of upstream items to prefetch so that fresh items are
ready to be mapped when a previous {@code SingleSource} terminates.
The operator replenishes after half of the prefetch amount has been consumed
and turned into {@code SingleSource}s.
@return a new Observable instance
@see #concatMapSingle(Function, int)
@since 2.2