code: @CheckReturnValue
      @BackpressureSupport(BackpressureKind.ERROR)
      @SchedulerSupport(SchedulerSupport.NONE)
      public final <B> Flowable<Flowable<T>> window(Publisher<B> boundaryIndicator, int bufferSize) {
          ObjectHelper.requireNonNull(boundaryIndicator, "boundaryIndicator is null");
          ObjectHelper.verifyPositive(bufferSize, "bufferSize");
          return RxJavaPlugins.onAssembly(new FlowableWindowBoundary<T, B>(this, boundaryIndicator, bufferSize));
      }
docs: Returns a Flowable that emits non-overlapping windows of items it collects from the source Publisher
  where the boundary of each window is determined by the items emitted from a specified boundary-governing
  Publisher.
  <p>
  <img width="640" height="475" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/window8.png" alt="">
  <dl>
  <dt><b>Backpressure:</b></dt>
  <dd>The outer Publisher of this operator does not support backpressure as it uses a {@code boundary} Publisher to control data
  flow. The inner Publishers honor backpressure and buffer everything until the boundary signals the next element.</dd>
  <dt><b>Scheduler:</b></dt>
  <dd>This version of {@code window} does not operate by default on a particular {@link Scheduler}.</dd>
  </dl>
  
  @param <B>
  the window element type (ignored)
  @param boundaryIndicator
  a Publisher whose emitted items close and open windows
  @param bufferSize
  the capacity hint for the buffer in the inner windows
  @return a Flowable that emits non-overlapping windows of items it collects from the source Publisher
  where the boundary of each window is determined by the items emitted from the {@code boundary}
  Publisher
  @see <a href="http://reactivex.io/documentation/operators/window.html">ReactiveX operators documentation: Window</a>