code: @BackpressureSupport(BackpressureKind.FULL)
      @CheckReturnValue
      @SchedulerSupport(SchedulerSupport.NONE)
      public static <T> Flowable<T> concatEager(Iterable<? extends SingleSource<? extends T>> sources) {
          return Flowable.fromIterable(sources).concatMapEager(SingleInternalHelper.<T>toFlowable());
      }
docs: Concatenates a sequence of SingleSources eagerly into a single stream of values.
  <p>
  <img width="640" height="319" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/Single.concatEager.i.png" alt="">
  <p>
  Eager concatenation means that once a subscriber subscribes, this operator subscribes to all of the
  source SingleSources. The operator buffers the values emitted by these SingleSources 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.</dd>
  <dt><b>Scheduler:</b></dt>
  <dd>This method does not operate by default on a particular {@link Scheduler}.</dd>
  </dl>
  @param <T> the value type
  @param sources a sequence of SingleSource that need to be eagerly concatenated
  @return the new Flowable instance with the specified concatenation behavior