code: public static <T> Flux<T> mergeSequential(Publisher<? extends Publisher<? extends T>> sources) {
  		return mergeSequential(sources, false, Queues.SMALL_BUFFER_SIZE,
  				Queues.XS_BUFFER_SIZE);
  	}
docs: Merge data from {@link Publisher} sequences emitted by the passed {@link Publisher}
  into an ordered merged sequence. Unlike concat, the inner publishers are subscribed to
  eagerly. Unlike merge, their emitted values are merged into the final sequence in
  subscription order.
  <p>
  <img class="marble" src="doc-files/marbles/mergeSequentialAsyncSources.svg" alt="">
  
  @param sources a {@link Publisher} of {@link Publisher} sources to merge
  @param <T> the merged type
  
  @return a merged {@link Flux}, subscribing early but keeping the original ordering