code: @CheckReturnValue
      @BackpressureSupport(BackpressureKind.FULL)
      @SchedulerSupport(SchedulerSupport.NONE)
      public final <R> Flowable<R> concatMapEagerDelayError(Function<? super T, ? extends Publisher<? extends R>> mapper,
              int maxConcurrency, int prefetch, boolean tillTheEnd) {
          ObjectHelper.requireNonNull(mapper, "mapper is null");
          ObjectHelper.verifyPositive(maxConcurrency, "maxConcurrency");
          ObjectHelper.verifyPositive(prefetch, "prefetch");
          return RxJavaPlugins.onAssembly(new FlowableConcatMapEager<T, R>(this, mapper, maxConcurrency, prefetch, tillTheEnd ? ErrorMode.END : ErrorMode.BOUNDARY));
      }
docs: Maps a sequence of values into Publishers and concatenates these Publishers eagerly into a single
  Publisher.
  <p>
  Eager concatenation means that once a subscriber subscribes, this operator subscribes to all of the
  source Publishers. The operator buffers the values emitted by these Publishers and then drains them in
  order, each one after the previous one completes.
  <dl>
  <dt><b>Backpressure:</b></dt>
  <dd>Backpressure is honored towards the downstream, however, due to the eagerness requirement, sources
  are subscribed to in unbounded mode and their values are queued up in an unbounded buffer.</dd>
  <dt><b>Scheduler:</b></dt>
  <dd>This method does not operate by default on a particular {@link Scheduler}.</dd>
  </dl>
  @param <R> the value type
  @param mapper the function that maps a sequence of values into a sequence of Publishers that will be
  eagerly concatenated
  @param maxConcurrency the maximum number of concurrent subscribed Publishers
  @param prefetch
  the number of elements to prefetch from each source Publisher
  @param tillTheEnd
  if true, exceptions from the current Flowable and all the inner Publishers are delayed until
  all of them terminate, if false, exception from the current Flowable is delayed until the
  currently running Publisher terminates
  @return the new Publisher instance with the specified concatenation behavior
  @since 2.0