code: public final ParallelFlux<T> parallel(int parallelism, int prefetch) {
  		return ParallelFlux.from(this,
  				parallelism,
  				prefetch,
  				Queues.get(prefetch));
  	}
docs: Prepare this {@link Flux} by dividing data on a number of 'rails' matching the
  provided {@code parallelism} parameter, in a round-robin fashion and using a
  custom prefetch amount and queue for dealing with the source {@link Flux}'s values.
  Note that to actually perform the work in parallel, you should call
  {@link ParallelFlux#runOn(Scheduler)} afterward.
  
  <p>
  <img class="marble" src="doc-files/marbles/parallel.svg" alt="">
  
  @param parallelism the number of parallel rails
  @param prefetch the number of values to prefetch from the source
  
  @return a new {@link ParallelFlux} instance