reactor-core/src/main/java/reactor/core/publisher/Flux.java
public static <I> Flux<I> mergeSequential(Iterable<? extends Publisher<? extends I>> sources) {
		return mergeSequential(sources, false, Queues.SMALL_BUFFER_SIZE,
				Queues.XS_BUFFER_SIZE);
	}
public static <I> Flux<I> mergeSequential(Iterable<? extends Publisher<? extends I>> sources) {
		return mergeSequential(sources, false, Queues.SMALL_BUFFER_SIZE,
				Queues.XS_BUFFER_SIZE);
	}
Merge data from {@link Publisher} sequences provided in an {@link Iterable}
into an ordered merged sequence. Unlike concat, sources 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/mergeSequentialVarSources.svg" alt="">

@param sources an {@link Iterable} of {@link Publisher} sequences to merge
@param <I> the merged type

@return a merged {@link Flux}, subscribing early but keeping the original ordering