code: @BackpressureSupport(BackpressureKind.FULL)
      @CheckReturnValue
      @SchedulerSupport(SchedulerSupport.NONE)
      public static <T> Flowable<T> concat(Iterable<? extends MaybeSource<? extends T>> sources) {
          ObjectHelper.requireNonNull(sources, "sources is null");
          return RxJavaPlugins.onAssembly(new MaybeConcatIterable<T>(sources));
      }
docs: Concatenate the single values, in a non-overlapping fashion, of the MaybeSource sources provided by
  an Iterable sequence.
  <p>
  <img width="640" height="526" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/Maybe.concat.i.png" alt="">
  <dl>
  <dt><b>Backpressure:</b></dt>
  <dd>The returned {@code Flowable} honors the backpressure of the downstream consumer.</dd>
  <dt><b>Scheduler:</b></dt>
  <dd>{@code concat} does not operate by default on a particular {@link Scheduler}.</dd>
  </dl>
  @param <T> the value type
  @param sources the Iterable sequence of MaybeSource instances
  @return the new Flowable instance