src/main/java/io/reactivex/Observable.java
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final <R> Observable<R> concatMapMaybeDelayError(Function<? super T, ? extends MaybeSource<? extends R>> mapper) {
        return concatMapMaybeDelayError(mapper, true, 2);
    }
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final <R> Observable<R> concatMapMaybeDelayError(Function<? super T, ? extends MaybeSource<? extends R>> mapper) {
        return concatMapMaybeDelayError(mapper, true, 2);
    }
Maps the upstream items into {@link MaybeSource}s and subscribes to them one after the
other terminates, emits their success value if available and delaying all errors
till both this {@code Observable} and all inner {@code MaybeSource}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 concatMapMaybeDelayError} 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 MaybeSource}s
@param mapper the function called with the upstream item and should return
a {@code MaybeSource} to become the next source to
be subscribed to
@return a new Observable instance
@see #concatMapMaybe(Function)
@see #concatMapMaybeDelayError(Function, boolean)
@since 2.2