code: public final <R> Flux<R> flatMapMany(Function<? super T, ? extends Publisher<? extends R>> mapper) {
  		return Flux.onAssembly(new MonoFlatMapMany<>(this, mapper));
  	}
docs: Transform the item emitted by this {@link Mono} into a Publisher, then forward
  its emissions into the returned {@link Flux}.
  
  <p>
  <img class="marble" src="doc-files/marbles/flatMapMany.svg" alt="">
  
  @param mapper the
  {@link Function} to produce a sequence of R from the the eventual passed {@link Subscriber#onNext}
  @param <R> the merged sequence type
  
  @return a new {@link Flux} as the sequence is not guaranteed to be single at most