code: public final <R> Mono<R> flatMap(Function<? super T, ? extends Mono<? extends R>>
  			transformer) {
  		return onAssembly(new MonoFlatMap<>(this, transformer));
  	}
docs: Transform the item emitted by this {@link Mono} asynchronously, returning the
  value emitted by another {@link Mono} (possibly changing the value type).
  
  <p>
  <img class="marble" src="doc-files/marbles/flatMapForMono.svg" alt="">
  
  @param transformer the function to dynamically bind a new {@link Mono}
  @param <R> the result type bound
  
  @return a new {@link Mono} with an asynchronously mapped value.