code: public final <U> ParallelFlux<U> map(Function<? super T, ? extends U> mapper) {
  		Objects.requireNonNull(mapper, "mapper");
  		return onAssembly(new ParallelMap<>(this, mapper));
  	}
docs: Maps the source values on each 'rail' to another value.
  <p>
  Note that the same mapper function may be called from multiple threads
  concurrently.
  
  @param <U> the output value type
  @param mapper the mapper function turning Ts into Us.
  
  @return the new {@link ParallelFlux} instance