code: public final <U> Mono<T> timeout(Publisher<U> firstTimeout) {
  		return onAssembly(new MonoTimeout<>(this, firstTimeout, "first signal from a Publisher"));
  	}
docs: Signal a {@link TimeoutException} in case the item from this {@link Mono} has
  not been emitted before the given {@link Publisher} emits.
  
  <p>
  <img class="marble" src="doc-files/marbles/timeoutPublisher.svg" alt="">
  
  @param firstTimeout the timeout {@link Publisher} that must not emit before the first signal from this {@link Mono}
  @param <U> the element type of the timeout Publisher
  
  @return an expirable {@link Mono} if the item does not come before a {@link Publisher} signals