src/main/java/io/reactivex/Observable.java
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final <R> Observable<R> concatMapSingle(Function<? super T, ? extends SingleSource<? extends R>> mapper) {
        return concatMapSingle(mapper, 2);
    }
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final <R> Observable<R> concatMapSingle(Function<? super T, ? extends SingleSource<? extends R>> mapper) {
        return concatMapSingle(mapper, 2);
    }
Maps the upstream items into {@link SingleSource}s and subscribes to them one after the
other succeeds, emits their success values or terminates immediately if
either this {@code Observable} or the current inner {@code SingleSource} fail.
<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 concatMapSingle} 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
@return a new Observable instance
@see #concatMapSingleDelayError(Function)
@see #concatMapSingle(Function, int)
@since 2.2