code: @CheckReturnValue
      @SchedulerSupport(SchedulerSupport.NONE)
      @BackpressureSupport(BackpressureKind.FULL)
      public final Completable concatMapCompletable(Function<? super T, ? extends CompletableSource> mapper, int prefetch) {
          ObjectHelper.requireNonNull(mapper, "mapper is null");
          ObjectHelper.verifyPositive(prefetch, "prefetch");
          return RxJavaPlugins.onAssembly(new FlowableConcatMapCompletable<T>(this, mapper, ErrorMode.IMMEDIATE, prefetch));
      }
docs: 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
  @param prefetch The number of upstream items to prefetch so that fresh items are
  ready to be mapped when a previous {@code CompletableSource} terminates.
  The operator replenishes after half of the prefetch amount has been consumed
  and turned into {@code CompletableSource}s.
  @return a new Completable instance
  @see #concatMapCompletableDelayError(Function, boolean, int)
  @since 2.2