code: public static Mono<Void> when(Publisher<?>... sources) {
  		if (sources.length == 0) {
  			return empty();
  		}
  		if (sources.length == 1) {
  			return empty(sources[0]);
  		}
  		return onAssembly(new MonoWhen(false, sources));
  	}
docs: Aggregate given publishers into a new {@literal Mono} that will be fulfilled
  when all of the given {@literal sources} have completed. An error will cause
  pending results to be cancelled and immediate error emission to the returned {@link Mono}.
  <p>
  <img class="marble" src="doc-files/marbles/when.svg" alt="">
  <p>
  @param sources The sources to use.
  
  @return a {@link Mono}.