src/main/java/io/reactivex/Flowable.java
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    @BackpressureSupport(BackpressureKind.FULL)
    public final Completable concatMapCompletable(Function<? super T, ? extends CompletableSource> mapper) {
        return concatMapCompletable(mapper, 2);
    }
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    @BackpressureSupport(BackpressureKind.FULL)
    public final Completable concatMapCompletable(Function<? super T, ? extends CompletableSource> mapper) {
        return concatMapCompletable(mapper, 2);
    }
Maps the upstream items into {@link CompletableSource}s and subscribes to them one after the
other completes.
<p>
<img width="640" height="305" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/concatMap.png" alt="">
<dl>
<dt><b>Backpressure:</b></dt>
<dd>The operator expects the upstream to support backpressure. If this {@code Flowable} violates the rule, the operator will
signal a {@code MissingBackpressureException}.</dd>
<dt><b>Scheduler:</b></dt>
<dd>{@code concatMapCompletable} does not operate by default on a particular {@link Scheduler}.</dd>
</dl>
<p>History: 2.1.11 - experimental
@param mapper the function called with the upstream item and should return
a {@code CompletableSource} to become the next source to
be subscribed to
@return a new Completable instance
@see #concatMapCompletableDelayError(Function)
@since 2.2