code: public final Flux<T> sequential(int prefetch) {
  		return Flux.onAssembly(new ParallelMergeSequential<>(this,
  				prefetch,
  				Queues.get(prefetch)));
  	}
docs: Merges the values from each 'rail' in a round-robin or same-order fashion and
  exposes it as a regular Publisher sequence, running with a give prefetch value for
  the rails.
  
  @param prefetch the prefetch amount to use for each rail
  
  @return the new Flux instance